Substrate - The Roots of Success

INTRODUCTION

At the heart of every thriving planted aquarium is its substrate. Substrates serve multiple vital roles beyond merely anchoring plants. It acts as a nutrient reservoir, supports beneficial bacterial communities essential for filtration, and is a significant part of the aesthetics of your planted aquarium. With a myriad of options available, ranging from nutrient-rich active substrates to inert sands and gravels, selecting the right substrate can seem daunting. Each type offers unique advantages and suits different plant species, tank setups, and aquarist experience levels.

This guide is designed to simplify the substrate selection process, providing key insights into their properties, interactions with plant roots, water chemistry, and the microbial ecosystem. It aims to equip aquarists, whether seasoned or new to the hobby, with the knowledge needed to lay the foundation for a lush, vibrant underwater garden. Additionally, for those currently stuck with a sub-optimal substrate setup, this guide offers strategies for to improve it for the long-term health of your aquarium plants. 

There is not one single best substrate choice for everyone - rather it depends on what kind of planted aquarium you are building, your priorities, and your experience level. 

Root Nutrition

Not to be mistaken for seaweed (left), aquatic plants are true higher plants, not unlike these redwoods (right)

Aquatic plants are frequently confused with seaweed and traits of seaweed, such as exclusively absorbing nutrients from the water column, have been ascribed to aquarium plants for a long time. Seaweeds are a type of algae known as macroalgae while modern aquatic plants, or aquatic macrophytes, descended from terrestrial plants and adapted to life in water while preserving many land-based characteristics. These adaptations include a reliance on root systems for nutrient absorption, which is often significantly more efficient than leaf absorption due to their evolutionary heritage.

The substrate plays a crucial role in the nutrition of aquarium plants by supplying nutrients directly to their roots. This method is not only more direct but also prevents the potential chaos of releasing nutrients into the water column, where they can harm fish and shrimp, encourage unsightly algae growth, and compete with plants for resources like light, CO2, and space.

Certain aquatic plants, particularly those that grow in a rosette pattern, rely almost solely on their roots for nutrition, with little uptake from the water. Stem plants, which can be found free-floating, can only achieve their full potential when rooted in the substrate. Those unable to reach the substrate will sometimes grow "adventitious" roots to enhance nutrient absorption. Floating plants, such as duckweed or water lettuce, have roots that dangle in the water, efficiently absorbing nutrients and benefiting from access to atmospheric CO2, contributing to their rapid growth rate.

What does “nutrient” mean?

Plant nutrition encompasses is complex - meriting its own in-depth exploration. For the scope of this article, "nutrient" will refer specifically to "Mineral Elements" required for plant growth. While CO2 (and other inorganic carbon forms) is a crucial nutrient, constituting up to 90% of plant dry mass, we'll set it aside for now since it more than deserves its own article. The elemental and mineral nutrients we’ll be discussing make up the remaining 10% of plant biomass. Without these nutrients, your plants cannot thrive or even function properly. For all living things, these nutrients are absolutely essential.

What makes a particular substrate good?

Substrates serve important functions in not just the growth of your plants but also the overall health of the planted aquarium ecosystem. A good substrate needs to meet these requirements:

1. Provide nutrition for aquatic plant roots, offering a direct supply of essential nutrients necessary for their growth and health. 

2. Keep as many nutrients as possible sequestered in the substrate bed rather than dissolved in the aquarium water.  

3. Provide an appropriate environment for roots to grow and for root systems to expand as much as needed.

4. Provide a habitat for beneficial bacteria that can directly provide nutrition and balance many processes in an aquarium, not least of which is the nitrogen cycle.

5. Allow for sufficient water flow to keep the substrate bed well-oxygenated and keep supplying beneficial bacteria with the food sources they require.

6. Be dense enough to solidly anchor plants and to minimize large disruptions to the substrate bed itself.

   
   

Since it’s such a prominent part of any aquarium setup, substrates should also be aesthetically pleasing, or at least not detract from the aesthetics too much. It should also be appropriate for bottom-dwelling fish and shrimp - not too sharp that it injures them.  

SUBSTRATE TYPES

Each type of substrate—active, passive, and inert—brings unique benefits and considerations to the aquatic environment. Choosing the right substrate depends on the specific needs of the aquarium's plants and inhabitants, as well as the experience level and priorities of the aquarist. By carefully selecting and managing the substrate, you can create a balanced, healthy, and visually appealing underwater landscape.

The table below summarizes the different kinds of substrates and their suitability for planted aquariums.

ACTIVE SUBSTRATES - AQUA SOILS (or AQUASOILS)

An Amano shrimp hanging out on an aqua soil substrate

Active substrates are also called aqua soils and are a fantastic choice for prioritizing plant growth and establishing the ideal water chemistry for a planted aquarium. These are a powerful addition to your tank that have have profound effects on nearly everything.

Aqua Soils Actively Alter Water Chemistry

We call these “active substrates” because they “actively” alter water chemistry by participating in chemical reactions designed to simulate the "average" natural environment of densely vegetated aquatic habitats.

If you’re a beginner, be mindful and make sure you understand the product before using it. Most of these are considered a “high-tech” substrate, intended for use in planted aquariums with CO2 fertilization and good lighting. They can be harmful or pointless in a low-tech setup since plants will be constrained by carbon availability and lighting far before running out of nutients.

Many aqua soilds contain a lot of soluble substances that if left unused by plants, can hurt your fish and shrimp, feed algae, or overwhelm the system so much that it crashes.

These substrates tend to make your water slightly more acidic and softer. That can reduce the availability of compounds like bicarbonate that some low-tech plants such as (eg. Hydrilla, Elodea, and Cabomba) depend on and may negatively impact their health without CO2 supplementation.

Composition

Made from kiln-baked balls of mostly soil and clay, these aquarium substrates are designed to, and usually successfully soften the water and lower its pH, to replicate the soft, acidic waters found in natural vegetated aquatic habitats.

They are also quite nutrient-rich and will usually have a variety of inorganic plant nutrients (such as ammonia, nitrate, or phosphate) that are easily accessible by the roots of aquatic plants. These compounds can also dissolve into the water column in a process known as “leaching” and it is especially important to be aware of whether or not a particular product will leach ammonia. 

The following table breaks down the most common components of aqua soils and their effects on water chemistry and aquatic plant growth

We’ve had a few customers creating their first high-tech planted aquarium and inadvertantly used a product that released copious amounts of ammonia, making it impossible to introduce animals for a long time. One customer used so much that he burned his plants and took two weeks of constant water changes to get the ammonia level low enough for the tank to begin cycling.

If you are using active substrates - before introducing fish or shrimp, be sure to test the water for:

• Ammonia (NH3)

• pH

• Alkalinity (KH or "Carbonate Hardness")

• General hardness

  
  

With some hard work, his tank turned out beautifully in the end.  

Origin and Purpose:

Takashi Amano, a pioneer whose contributions have significantly shaped modern aquascaping, popularized the use of aquasoils to cultivate his stunning aquascapes. They were an important part of his philosophy and approach, to replicate the conditions of natural ecosystems inside the aquarium by adjusting CO2 concentration, lighting intensity, and water chemistry.

Aquasoils were designed to adjust and, to a certain extent, buffer water parameters and, together with his other techniques, had an astounding effect on plant growth. Hobbyists embraced aquasoils in droves because, frankly, they worked. To this day, they remain the dominant substrate seen in the prominent aquascapes featured in magazines and across social media platforms. They are a mainstay in the planted aquarium industry.

Active substrates are marketed by many respected and well-known names in the planted aquarium industry and each company often has multiple products available in a dizzying range of options each tailored for a specific use case so pick one that's best for you.

Types of Aquasoils:

Due to the large variety of products on the market, this are some general trends I’ve noticed in practice and from speaking with manufacturers and distributors. There will always be a product that bucks the trend - so treat this as a rule-of-thumb:

The table below summarizes some of the characteristics of two broad types of aqua soils

The pH 6.5 version is often more stable, both chemically and structurally. It releases nutrients slowly and is less prone to break down over time, making it a more forgiving choice for beginners. This more subdued and gradual influence on water chemistry, coupled with its structural longevity, makes it ideally suited for those new to planted tanks.

Many products that target a pH range around 5.5 are compounded to be more reactive, swiftly altering the water's pH to create specific conditions favored by certain plants and fish. These are often the domain of more experienced aquarists or those constructing specific biotopes, where a rapid establishment of the desired water parameters is important.

Its high reactivity and tendency to disintegrate relatively quickly make it generally difficult to recommend for beginners. These powerful substrates are especially popular among content creators who (like Amano himself)  require a visually mature aquascape in a short timeframe but often do not plan on maintaining the setup long-term - lest they wish to have hundreds of aquariums. 

Usage Considerations:

Closeup of the distinctive round granules of an aqua soil (Fluval Stratum)

As you might have guessed, because they are manufactured and offer a lot of alluring advantages, aqua soils tend to be more expensive, and some beginners just assume that they are better because they cost more. While their benefits are significant, especially in setups with CO2 supplementation and lighting to match, it's critical to assess whether they align with the needs and goals of your planted aquarium as a whole.

Notably, without CO2 powering rapid plant growth, the nutrients released by aquasoils can lead to algae issues, underscoring the importance of matching your substrate to your priorities. 

Additionally, aquasoils tend to lower pH which makes bicarbonate (HCO3-) less available. This reduces the amount of bicarbonate available for the few aquarium species that can use it when CO2 concentrations are low (such as Vallisneria, Egeria, and some Cryptocornes). While this can benefit species that cannot use bicarbonate (eg. Eleocharis, “HC Cuba”, Rotala rotundifolia) since bicarbonate gets converted to carbonic acid which decomposes into CO2 and water. While producing CO2, a lot of it gets lost to the atmosphere. This also reduces the KH buffering capacity of the water, making it more likely to experience pH swings.

Given the composition and specialized nature of aqua soils, it's important to do some research and read product labels carefully before making a selection. If you want to recreate a specific environment such as the Amazon Basin, you should pick a product designed to do so. If you are more inclined toward a general planted aquarium setup, you’ll best be served by something milder.

Recommendations:

For those new to planted aquariums, I would generally advise caution and recommend that you get to know the different products. While some are very potent, with side effects to match, others are pretty harmless. Fluval’s Stratum, for example, is user-friendly and laid-back enough that I’ve never even had to think about it. My plants have thrived in it and tanks set up for years show no signs of disintegration.

These more forgiving aquasoils are a fantastic medium for growing amazing plants without the risk of surprises or harm to animals. They are well-suited to someone just starting - especially if you are using CO2.

Some newer aquasoils are designed to maintain optimal pH and hardness levels for longer periods and are generally more stable. They are engineered to release nutrients at a more predictable pace over time, ensuring a constant supply for plant growth while keeping water chemistry stable. Some industry players have recognized the issues with certain products and are focusing on improvements to make them more beneficial for aquarists aiming for long-term planted aquarium setups.

PASSIVE SUBSTRATES:

Unlike active aquasoils,  passive substrates are not endowed by their creator with organic matter or soluble mineral salts. When new, they are essentially a blank slate and have no measurable effect on anything. They do not “actively” participate in chemical reactions and their value for aquarium plants is due to their immense potential to retain nutrients acting as a resevoir.

Rather than being a direct source, they manage and concentrate the nutrients that already exist in a system, retaining them in the substrate bed where they are available to the roots of aquarium plants. The retain and enhance the effects of root tabs by retaining the nutrients and distributing them throughout the bed.

They do not affect pH or hardness, nor do they leach compounds such as ammonia. This stability can be an invaluable consideration for beginners or anyone else who prioritizes predictability and nutrient management in their planted aquariums. It's a great way to make sure you don't end up feeding an algae bloom before your plants get off the ground.

I have used these extensively, with and without CO2, and have never noticed any nutrient limitations. Whether or not you fertilize with CO2, I would highly recommend one or more of these options.

A Checking Account for Nutrients

Like this ion exchange resin from a water filter, high CEC passive substrates can attract nutrients and hold on to them

Passive substrates do not come "charged" with nutrients but rather excel in their management due to their high “Cation Exchange Capacity” (CEC): a concept that many terrestrial gardeners may already be familiar with. This essentially means that they have an affinity for “cations” - positively charged substances such as potassium, calcium, magnesium, and ammonium (K+, Ca2+, Mg2+, NH4+ respectively).

This ability to absorb nutrients from various sources — the water column, root tabs, or decomposing organic matter — allows them to act like a nutrient bank, holding these substances close to the root zone of plants and out of your water column.

What about anions like Nitrate and Phosphate?

While high CEC substrates specialize in attracting and retaining cations, they do not repel anions which are essential for plants. A high CEC does not mean a substance has a negative charge, they are neutral to begin with and anions like nitrate and phosphate are as available as ever and free to move about the substrate.

Far from being a problem,  these substrates often have an indirect, positive impact on the supply and bioavailability of these vital nutrients. Their positive effects on root system health and microbial activity can improve a plant's ability to take in all nutrients. And their massive surface areas are densely colonized by nitrifying bacteria which rapidly process retained NH4+ into a steady supply of nitrate for plant roots. 

In aquariums with fish, anionic nutrients are rarely in short supply. If a dense rapidly growing plants fed by CO2 and good lighting does manage to deplete nitrate and phosphate, liquid fertilizers are very effective for supplementing them.

The initial (nutrient) deposit

Root tabs come in all shapes and forms. We recommend using them at least once to "charge up" a new bed of high-CEC passive substrate

The quickest way to charge up a passive substrate is with root tabs, which should be used sparingly. As long as there are nutrients available in your aquarium, the substrate can recharge itself again and again. By the time a tank is well established, the substrate will be mature, and a steady supply of nutrients from detritus and the water column will continuously keep the substrate charged. If there are fish, I use root tabs just once with these substrates, usually after the plants become a bit more established. There are usually sufficient nutrients in tap water to keep the plants fed at the beginning while they acclimate. Excessive amounts of nutrients at the very start will be eagerly used by algae - which you need to hold back until the plants take off. If algae take off first, they will shift the balance of a planted aquarium in their favor - which will be difficult to reverse. It’s better to err against feeding algae since a nutrient-deficient plant almost always recovers after the deficiency is addressed but it will be difficult, if not impossible to reduce the algae population if it establishes itself before your plants do. 

Nutrient Diffusion and Mobility: The Sponge Effect

High CEC substrates in an aquarium environment with plenty of watery solvent to go around, can help distribute nutrients from one part of the substrate bed to where they are being depleted by plant roots. Very often, the exposed cation binding site that remains after an ion is transported into the plant will be filled with another cation from a nearby area with a higher concentration or from the water column. This has the effect of making the nutrients mobile and shuttling them around the substrate bed to the roots of plants - wherever they are. As a result, you do not have to be as precise with root tab placement, while we recommend that you put them close to the roots, if you miss the exact spot, the nutrients will eventually find their way to where they are needed. A highly localized supply of nutrients (from a root tab for instance) will be spread around the general area by a high CEC substrate - much like a drop of water on the surface of a sponge. 

Maturation

After an initial root tab charge, the high CEC of passive substrates allows them to absorb and retain these nutrients, making them available for plant uptake. After being in an aquarium for an extended period, a passive substrate matures, significantly enhancing its ability to support aquatic plant growth. 

Organic detritus decomposes into humus in the substrate. This humus, rich in organic matter, can enhance the substrate's innate CEC. Humus has an even higher CEC (and is often a component of aqua soils), which further increases the substrate's ability to hold onto and exchange nutrients. The texture and porosity of these substrates hold onto particles of humus exceedingly well. This natural enrichment process helps to maintain and even improve nutrient retention, maintaining a rich environment for the roots of plants. Assuming there’s fish and fish food, even rapid CO2-accelerated growth does not frequently require further fertilization - especially if appropriate liquid fertilizers are used as required.

Be careful what you clean! Detritus on your substrate makes it mature and better for growing plants. Clean if you must, but leave some gunk for the plants!

In very densely planted, rapidly growing setups that have a low fish bioload and little to no water column enrichment, the nutrient reserves in the substrate may be depleted faster, potentially requiring additional fertilization. In this case, periodically assess the condition of your plants. Signs of nutrient deficiency or a decline in growth rates may indicate the need for re-fertilization. Introducing more root tabs or applying liquid fertilizers will replenish the nutrient content in the substrate, ensuring that plants continue to thrive.

CEC and Active Substrates

Active Substrates - such as Aqua soils have extremely high CEC and mature well. Once the nutrients run out, they are better than anything at recharging themselves. The main concern about aqua soils is the nutrients they ship with that can leach into the water column, giving the aquarist less control over nutrient levels. They also change water chemistry which, though usually welcome, can be problematic in some cases. The main difference between the two is that Passive Substrates are not “dosed” to being with and do not change water chemistry directly. 

Common Products:

There are many types of high CEC passive substrates but a few stand out for their availability and ease of use. They are the ones that I’ve had the most experience with and can personally recommend.

Flourite (SeaChem)

Flourite is a proprietary blend of clays containing a wealth of minerals, including aluminum, iron, magnesium, and potassium. These minerals are, however, insoluble and sequestered in the structure of the clay itself so they are unavailable to plants out-of-the-bag. This is not immediately obvious by studying the label or advertising and is a prime example of why doing your homework is important. Like most passive substrates, Flourite starts empty but gets filled over time by root tabs, decaying organic material, and even nutrients in the water column. These substances, now safely stored in the substrate are readily available to your aquarium plants as needed. 

Flourite’s biggest advantage is the availability of different options. It is available in a classic reddish brown or in a charcoal black - the latter of which I prefer since it looks better in a planted aquarium. It is also available in a coarse or fine grain - which can be very helpful for both aesthetic and practical considerations

My favorite, for its aesthetics and overall usability, is the variety called “Flourite Black Sand” which has a very natural appearance and is easier to plant in.

Because fluorite is not very dense and is inherently porous, it does not compact and despite the finer grain, still allows for a good amount of water flow. It also has more surface area for nutrient exchange and for beneficial bacteria to colonize. Any burrowing creatures will also have an easier time doing so. Other aquascapers may favor the coarser version which is still pretty fine-grained by most measures and looks more like a fine gravel. It may allow for greater water flow but has less total surface area. I find the coarser type more difficult to plant in but it’s far from impossible - or even particularly difficult - the sand is just easier. Either type is well suited for even very high-tech aquariums and is entirely a personal preference. You can even mix them for the best of both worlds.

 Eco-Complete (Carib Sea)

Enlargement showing the grains of Eco-Complete in detail. Notice the extremely porous surface typical of basalt rock

A proprietary mineral blend based on crushed volcanic basalt, Eco-Complete is a popular option for planted aquariums and one that you really can’t go wrong with. It comes pre-colonized with heterotropic bacteria to jump-start many microbial processes and helps with the establishment of nitrifying bacteria (though they are not nitrifying bacteria). It’s designed to speed up the cycling process but is not “pre-cycled” so you will still need to cycle your tank before introducing the full population of fish and shrimp.

Eco-Complete includes minerals such as iron, calcium, magnesium, potassium, and other trace elements that are crucial for plant health and development. Unlike Flourite, these minerals are present in a form that is available for use by plants (at least according to the manufacturer). They may be in the form of insoluble compounds but are available to roots via biochemical processes or compounds with low solubility that mostly remain in the substrate bed. It does not contain organic matter or macronutrients such as ammonia, nitrate, or phosphate, and will have no practical effect on water chemistry. While it is not quite an “empty” bank account, it’s certainly not flush and we recommend at least one (conservative) round of root tab fertilization.

Like Fluorite, it has a very high CEC and will hold most of the nutrients in the substrate bed. It also boasts a vast surface area for beneficial bacteria to colonize which will promote overall ecosystem function as well as nourishing roots, and bioaccumulating nutrients. Once the aquarium has matured and the substrate has matured, it will stay well-charged without intervention. 

Eco-complete is available in one variety - a crushed basaltic rock with variable grain sizes and a vast amount of surface area. It is quite natural and aesthetically appealing in a planted tank. The grain sizes are variable and it will not compact - additionally, the basalt texture of eco-complete greatly aids water flow, which along with its porous nature, makes it perfect for dense colonies of productive micro-biota that are constantly being provided with fresh food and oxygen. True to its name, it does help the development of microbial processes that “complete” the ecosystem of your planted aquarium - at least as much as it can be considered complete while being contained in a glass box. 

Eco-Complete has a denser, sharper texture compared to Flourite, which might make it slightly more challenging to plant in without scraping the roots. However, I have no evidence at all that it actually damages the plant. It's heavy right out of the retail bag, offering a good workout, particularly for larger tanks. Unlike Flourite, Eco-Complete is sold pre-moistened, so there's no reduction in weight from drying. It's designed for immediate use, and its initial grayish, cloudy water will clarify over a few days as tiny particles settle or get filtered, leaving your aquarium beautifully clear.

Despite being less convenient, and more expensive, than Flourite, Eco-Complete is a fantastic choice for any planted aquarium - even with a lot of fast-growing plants, CO2, and plentiful lighting. It might be the best option if you prioritize the microbial processes in your substrate which not only benefit plants but the overall health of your aquarium and could, perhaps, reduce algae and other maintenance issues. The heterotrophic bacteria in the substrate may help waste management processes, possibly creating more humus faster - which would improve the nutritive value of the substrate as it matures. 

Arcillite

Extreme closeup of arcillite grains. They do not appear to be the same material - perhaps each contributing something useful. Notice the extremely fine pores.

A kiln-baked clay frequently used as hydroponic media, arcillite is occasionally sold for aquatic plant use (usually for ponds). It has various brand names including “Turface” and “Aquatic Planting Media”. It’s a fairly light reddish-brown color and is usually quite inexpensive. It’s quite light for a substrate material as it is an expanded material that is extremely porous, making it ideal for microbial colonization. It does not have any noticeable effect on water chemistry, and like Floruite and Eco-Complete, Arcillite has excellent CEC. 

Though it’s an obscure product for planted aquariums, I’ve had my fair share of experience using it and can vouch that it works well. Because it’s fairly inexpensive and comparatively light, we’ve used it extensively for lab testing things other than substrate. While it does an excellent job at growing plants, with results indistinguishable from similar substrates, I’m not a huge fan of the color and have not used it often, if at all for display aquariums. 

It’s fairly similar to Flourite Classic in terms of texture and consistency but is paler and noticeably less dense, which makes it trickier for anchoring plants, especially when newly planted. It works well after the roots have grown out. We’ve never had any unpleasant experiences and have used it extensively, more or less as a “control” since different batches have been very consistent. If the color and low density are not huge issues for you, I’d highly recommend checking it out. It’s significantly less expensive than the other two options. We used it in a form sold as “Aquatic Plant Media” by API. The packaging and size suggest that it was intended for ponds.

There have been some claims that this is just kitty litter. While they could feasibly be made from the same types of clay, arcillite is baked and expanded, and thus stable in water whereas kitty litter will fall apart and make a mess. There are some claims that cat litter works as a planted aquarium substrate but in my experience, it ends up being a goopy mess and I’d recommend against it unless you can fire it in a kiln first. 

Others:

Many other materials are stable in water, have a high CEC, and may be suitable as planted aquarium substrates, but they are not as well studied, or as easily available. Some are unattractive, others are too light to hold plants down, and some are not quite passive and will eventually alter your water chemistry. Some of these are intended as building materials that may not be suitable for aquarium use. If you’re adventurous, feel free to explore these as long as you practice some due diligence.  

Chemical Filtration

The principle of CEC is also the reason that many chemical filtration media work. Zeolites, for example, are microporous, aluminosilicate minerals known for their ability to exchange cations. In aquariums, they are used to remove (NH4+) from water, making zeolites a popular choice for emergency media in case of ammonia spikes as well as for ongoing water quality maintenance in fish-only aquariums. It can theoretically make a good substrate but is rarely used for that purpose, being generally more expensive and a dingy white color that may not be desirable for aesthetic reasons. 

Quick Note about General Hardness:

Because high CEC substrates sequester Mg and Ca ions, they can theoretically reduce the general hardness (GH) of the water but the effect is very weak and cannot usually be measured. It can essentially be disregarded. They do not affect carbonate hardness (also called alkalinity or KH). 

INERT SUBSTRATES - GRAVEL AND SAND

Extreme closeup of "Black Moon Sand" - a silica substrate that will not effect water chemistry, not is it inherent good for aquatic plants

Silica-based substrates, commonly in the form of gravel or sand can be referred to as “inert substrates” due to their minimal influence on water chemistry and are biotically inert - with no inherent effects on plant growth and a fairly minimal effect on the ecology of your planted aquarium as a whole. They were traditionally, the only substrates commonly used for fish-only aquariums and remain a popular choice for their aesthetic appeal that may be hard to find a replacement for, universal availability, and cost-effectiveness. 

In the the context of a planted aquarium, they present several challenges including nutrient availability, nutrient retention, and lack of microbial habitat. In the case of sand, there is a very unpleasant potential for compaction and the development of anaerobic pockets. That is not to say that it is impossible, or even difficult to create a vibrant and thriving plant community with these, there are just some challenges to keep in mind. Understanding these challenges and overcoming them is crucial for aquarists seeking to cultivate lush, vibrant aquatic gardens using these aesthetically unique, and often widely available materials as the substrate.

Lack of Nutrient Content

One of the primary drawbacks of silica-based inert substrates is their lack of inherent nutrients. Unlike active substrates specifically designed to support plant growth by providing essential minerals, inert substrates do not supply any nutrients to plants. This can lead to slower growth rates and potential nutrient deficiencies unless rectified by additional fertilization.

Aquarium gravel is available in colors no natural substrate is though it's not ideal for aquatic plants

Due to their low Cation Exchange Capacity, they are far less effective at keeping nutrients in the substrate bed and cannot distribute them where they are needed. Since they cannot store nutrients, these substrates rely on the water column or root tabs for nutrients. Root tabs in particular are very important and in contrast to high CEC substrates, they need to be used indefinitely (but not excessively). In addition, without the CEC to distribute nutrients within the substrate bed to where they are needed, root tabs need to be strategically placed as close to the source of nutrient uptake - the roots - as possible. Nutrients don’t easily spread out from their input site but rather dissolve and leach into the water column or remain undissolved within solid particles which eventually end up at the bottom of the substrate bed by gravity. 

Does not promote microbial activity

While gravel and sand can support plant growth by allowing for nutrient tabs to be placed near the roots, their contribution to the ecosystem's health is notably less than that of high CEC substrates due to poor nutrient retention and limited surface area. In such setups, reliance on biological filtration becomes crucial. Ensuring the presence of efficient, porous biological media can help maintain aquarium health. 

While beneficial microbes in the substrate itself, such as mycorrhizae and nutrient-concentrating bacteria, may be less prevalent, plants are resilient and can adapt if provided with sufficient carbon and light. Regular use of root tabs can further support their nutritional needs, ensuring long-term health and growth. Colored aquarium gravel, often coated with epoxy, presents a smooth surface that offers no extra surface area.

Compaction and Anaerobic Conditions in Sand

Silica sand, a widely favored inert substrate, is especially susceptible to compaction over time, particularly when the grains are fine and uniform. Being heavy and non-porous, it compacts tightly at the aquarium's bottom, restricting water flow and limiting oxygen access for plant roots and beneficial microorganisms. This compaction can get so bad that it causes anaerobic pockets to form, promoting harmful bacteria growth that potentially releases toxic hydrogen sulfide gas, posing a risk to plants and aquatic life.

Despite this, sand is common in planted aquariums for a variety of reasons and can certainly be used. To mitigate compaction without resorting to frequent substrate stirring—which risks harming your plants—one effective method is using a sand cap - a shallow, mostly aesthetic layer of sand over a more porous substrate like aqua soil, Flourite, or Eco-Complete. This can offer a balance between looks and functionality. Mixing sand with a coarser material can also introduce necessary aeration. While healthy plant roots naturally aerate the substrate, they may not always prevent anaerobic conditions, particularly in less-planted areas or among shallow-rooted species.

The Kuhli Loach can help stir up and aerate deep sand substrates to prevent compaction

For aquariums already established with a dense sand bed, identify any discolored areas or bubbles indicating anaerobic pockets. Carefully "pop" these pockets with tools like tweezers, proceeding slowly and gradually to avoid releasing harmful gases into the water. Then stick a lava rock or some other porous material to “plug” the pocket and ensure it does not happen again. This proactive approach prevents potential issues before they escalate. 

Enhancing water circulation can also promote substrate aeration and prevent anaerobic areas from developing (or redeveloping). Introducing burrowing creatures (like Malaysian trumpet snails, Corydoras catfish, and Khuli loaches) aids in substrate mixing. In deeper substrates, incorporating items like lava rocks at regular intervals can further ensure adequate aeration, complementing the efforts of burrowing fauna.

Rumor Alert: I don’t even know where this came from except I was asked about it before - do NOT use

earthworms! This must’ve been a cruel joke - earthworms cannot live for long inside an aquarium as they absorb oxygen through their skin and will drown. Their decomposing bodies will create more oxygen stress making the problem much worse!

Maturing Gracefully: better with time

Like a fine wine, substrates improve with time. Initially nutrient-poor, even inert substrates become more conducive to plant growth as they accumulate organic detritus, such as fish waste, decaying plant material, and uneaten food. This detritus eventually transforms into humus, markedly altering the substrate bed's characteristics. Humus acts both as a natural fertilizer and enhances nutrient retention. A thriving microbial community can break down complex organic materials into simpler forms, accessible to plants. 

While inert substrates initially lack significant Cation Exchange Capacity (CEC), the addition of organic matter can substantially boost their nutrient retention capabilities. As organic content rises, so does the substrate’s capacity to hold onto nutrients from the decay process, creating a more stable nutrient environment for plant roots. This underscores the dynamic evolution of aquarium ecosystems, where substrates mature over time to better nurture the life they support, becoming more like soil and less like gravel or sand. In addition, the organic matter itself is “surface area” for microbial colonization.

This transformation is particularly pronounced in high CEC passive substrates, which possess a greater surface area for microbial colonization and are even more adept at retaining organic detritus, improving with time. 

PRACTICAL CONSIDERATIONS

Substrate Depth

The depth of your aquarium substrate is pivotal for the root growth and overall health of your aquatic plants. A deeper substrate allows more room for roots, benefiting larger plants with extensive root systems. For example, rosette plants like Amazon swords and crypts, known for their sizable root networks, thrive in deeper substrate beds. Conversely, a shallower substrate is suitable for smaller plants, such as Micranthemum 'Monte Carlo' or Glossostigma elatinoides.

A sloped substrate, deeper at the back and shallower at the front, not only adds visual depth - making your aquarium seem larger and fuller, but also caters to the rooting needs of different plants. This arrangement allows for larger background plants to have the depth they require, while foreground and carpet plants (which typically have shallower roots) are positioned near the front. Additionally, sloping helps guide detritus to the front for easier removal.

The table below shows how much substrate you should reasonably have at the front and rear of your tank

For the front, a depth of 1” to 2” (2.5 to 5 cm) is ideal for shorter foreground species with shallower roots. 

The back should have a depth of at least 3” to 4” (about 7.5 to 10 cm), accommodating the taller plants' larger root systems.

Layering Substrates

Using a nutrient-rich substrate layer over an inert one can be the best of borth worlds

Incorporating a nutrient-rich layer beneath a decorative top layer is a widely recommended strategy that marries aesthetic appeal with practical nutrition. This approach enables you to cap a nutrient-dense substrate, which might otherwise leach into the water column, with an inert layer. Such a setup provides aquatic plants with the essential nutrients they need while minimizing any potential disruptions to water chemistry. This is the best way to combine the aesthetics of an inert substrate with the benefits of a proper planted aquarium substrate. You can have your cake and eat it too. 

Example base layers

Aqua soils or Active Substrates: These are nutrient-dense layers often serving as the foundation for plant growth. Specifically formulated for aquatic environments, they support plant health and are safe for all tank inhabitants. Capping them can slow leaching and mitigate water chemistry effects.

Peat: A nutrient-rich material found in many aqua soils (and natural bogs), peat can significantly lower pH and soften water. However, if disturbed, it can clog filters and leach tannins and fulvic acids, darkening the water. Capping peat with an inert substrate mitigates these effects, preventing it from entering the filtration system and maintaining water clarity.

Laterite: An iron-rich choice ideal for plants needing high iron levels. Washing laterite can be challenging, and its fine particles may cloud the water, a concern easily addressed by covering it with another substrate.

When layering substrates, ensure both the base and cap layers are safe for aquarium use. This means they should be devoid of harmful chemicals, such as pesticides or artificial fertilizers, dyes, and sharp edges that could endanger tank residents. Opt for products specifically designed for aquariums or those in their natural, unaltered state.

Substrates and Aquascaping - Aesthetic and Artistic Considerations:

Substrates aren't just the foundation of your aquarium's ecosystem; they're the canvas for your planted aquarium. The choice of substrate impacts not only the health of your aquatic plants and animals but also the overall aesthetic appeal of your tank. Here's how the right substrate can elevate your aquascaping from ordinary to extraordinary.

Color and Texture

The color and texture of your substrate play pivotal roles in the visual impact of your aquascape. Darker substrates make the colors of fish and plants pop, while lighter ones can brighten up the entire tank and give it a more open feel. Texturally, fine sand offers a smooth, serene landscape, and is popular for use in unplanted areas or as a cap. This is entirely up to you and your aquascaping vision. 

Layering and Contouring

A thin layer of sand can be a beautiful way to create variety and visual separation between aquascape elements

Substrate isn't just to be laid flat. By varying the depth and creating contours, you can add dimensions to your aquascape. A gentle slope from back to front not only enhances the sense of depth but also showcases plants and decor at different heights, making your setup more dynamic. You can use it to define hills or valleys. Experiment with it.

Focal Points

Strategically placed substrate can direct the viewer's eye to the focal points of your aquascape. Whether it's a dramatic mountainous setup using coarse gravel or a peaceful beachfront scene with fine sand, the substrate you choose sets the stage for these key features. A favorite is using white sand to create a “path” through the underwater forest - which can be very dramatic. 

Harmony with Hardscape

The substrate should complement your hardscape—rocks and driftwood—not compete with it. Choose a substrate that contrasts in color or texture to make your hardscape elements stand out, or one that blends seamlessly for a more unified look. It depends on what you want, but keep it in mind.

Consider the Inhabitants

While aesthetics are important, remember that the substrate must suit the needs of your tank's inhabitants. Some fish and plants have specific substrate preferences, so choose a type that supports their health and well-being.

A WORD ABOUT SOIL

In "Ecology of the Planted Aquarium," Dr. Diana Walstad advocates for using soil as a substrate, presenting a solid case for its effectiveness within a specific, low-tech approach. While her methodology yields positive results, it's not universally suitable for all aquascapers. The challenge lies in soil's tendency to turn into mud when mixed with water, particularly if the sand cap intended to contain it is disturbed, leading to significant maintenance issues.

Soil acts as a hyper-active substrate, rapidly releasing nutrients into the water. This can cause drastic shifts in water chemistry, elevate ammonia levels, and provoke aggressive algae growth, all of which are difficult for beginners to manage. Additionally, soil tends to compact over time, making it potentially hazardous and the heavy sand cap does not help.

Given these challenges, newcomers to aquascaping might find managing soil's dynamic nature daunting. Given the complexity of maintaining a planted aquarium, starting with a commercially prepared substrate designed for aquarium plants might be the most pragmatic path to ensuring success.

WHEN DOES SUBSTRATE NOT MATTER?

In an aquarium with a lot of hardscape and plants (such as moss) growing on the hardscape, the substrate ins't as important but you should still choose one that is decorative and ecologically beneficial.

If you are creating a planted aquarium with exclusively hardscape and plants attached to hardscape - epiphytes - such as aquatic moss, Riccia, and Anubias, you don’t have to worry about substrate nutrition. Most of these plants either do not have roots or have evolved to use their root tissue to absorb nutrients from the water column. You should, however, still choose some decorative covering for the bottom of your tank for aesthetics and to make your fish comfortable. Nonetheless, the choice is far less consequential and you can use a shallow (½”) layer or whatever you like. Consider the substrate’s other properties like how it influences water chemistry or the ecosystem of your aquarium. Anubias roots will continue to grow downward toward the substrate and they do better if they can get to it but they can do very well just by feeding from the water column. 

CLOSING REMARKS AND REFERENCES

The substrate in a planted aquarium plays a pivotal role beyond just covering the bottom; it's essential for ecosystem health, plant health, and aesthetic charm. It nourishes plant growth, ensures stable anchorage, and improves water quality. Choosing the right substrate can lead to a lush planted tank and add visual appeal to your aquascape. Understanding the different substrate options, along with their advantages and limitations, is key. Familiarizing yourself with the basic principles of how substrates impact your aquarium ecosystem is invaluable. For both experienced aquarists and beginners, selecting the appropriate substrate is crucial. It can transform your aquascaping, boosting the health of your plants and the overall well-being of your aquarium.

References

Walstad, D. (2013). Ecology of the Planted Aquarium: A Practical Manual and Scientific Treatise for the Home Aquarist. Echinodorus Publishing. This book provides an in-depth exploration of using soil as a substrate in planted aquariums, along with the ecological considerations for creating a balanced aquarium ecosystem.

Horst, K., & Kipper, H. (2007). The Optimum Aquarium: For the Installation and Furnishing of Aquariums. Kasparek Verlag. This guide offers insights into aquarium setup and maintenance, including substrate choice and its impact on plant health and water chemistry.

James, P. (1997). The Planted Aquarium: A Practical Guide. TFH Publications. This practical guide discusses various aspects of setting up and maintaining a planted aquarium, including the selection of substrates and their role in plant nutrition and growth.

Rataj, K., & Horeman, T.J. (1977). Aquarium Plants: Their Identification, Cultivation, and Ecology. TFH Publications. This book includes information on the natural habitats of aquatic plants and how to replicate these environments in an aquarium, with emphasis on the importance of substrate.

Scheurmann, E. (1985). The New Aquarium Handbook: Everything About Setting Up and Taking Care of a Freshwater Aquarium. Sterling Publishing Co., Inc. This handbook provides a comprehensive overview of freshwater aquarium setup, including the selection of substrates and their effects on aquarium health.

Further Reading

Amano, T. (1994). Nature Aquarium World: Book One. TFH Publications, Inc. This book is the first in a series that introduces the concept of the nature aquarium, showcasing Amano's philosophy and techniques for creating beautifully planted aquariums that mimic natural ecosystems.

Amano, T. (1997). Nature Aquarium World: Book Two. TFH Publications, Inc. The second book in the series continues to explore the nature aquarium concept, providing readers with further inspiration and practical advice on creating their own aquatic landscapes.

Amano, T. (1997). Nature Aquarium World: Book Three. TFH Publications, Inc. The final book in the trilogy offers more advanced insights into the art and science of aquascaping, featuring a wide range of stunning aquarium setups and detailed information on plant and fish species.

Amano, T. (2011). ADA Nature Aquarium: Complete Works 1985-2009. Aqua Design Amano Co., Ltd. This comprehensive volume compiles Takashi Amano's works over a span of 24 years, highlighting his evolution as an aquascaper and the development of the nature aquarium aesthetic. It includes hundreds of photographs of his aquarium designs, along with essays and reflections on his approach to aquascaping.