The Celestial Pearl Danio and Micro Rasboras
Tiny Titans of the Planted Tank: The Rise of Celestial Pearl Danio, The Emerald Dwarf Rasbora and the "true" Micro Rasboras
For those that believe that there are some fish that should never be kept in an aquarium; in particular, large species with large habitats used to open-water hunting or seasonal migration, these tiny cyprinids are the polar opposite. Hailing from small ponds in South East Asia, this loosely related group of fish that have only recently become widely available are happier staying wherever there’s food and weaving their way among lush vegetation in small groups looking for tiny invertebrates. The are masters of the aquarium microhabitat and happy to live their entire lives within a confined space, as long as that space is stocked densely with plants!
Introduction
Celestial Pearl Danios (Danio margaritatus, also known as CPD or "Galaxy Rasbora") and Emerald Dwarf Rasboras (Danio erythromicron, historically called a rasbora) are shining stars that have rapidly become popular in the planted aquarium community. Discovered in 2006 in Myanmar, CPDs dazzle with their starry patterns and vibrant colors. Similarly, Emerald Dwarf Rasboras, known since 1918 (but only gaining popularity in the mid-2000s), boast striking colors and patterns. Both species, alongside true Micro Rasboras like Boraras brigittae and Boraras urophthalmoides, thrive in densely planted aquariums, showcasing their bright colors and dynamic behavior. Because of their small size, they have become wildly popular for small and nano tanks in particular.
Originating from the vegetated ponds of Southeast Asia, these fish are absolutely miniscule, often not exceeding an inch, making them ideal for tanks as small as 5 gallons, although 10 gallons or larger is recommended for a small shoal. Their preference for lush planted environments over sparsely decorated spaces is notable; in the lush setup of a planted aquarium, not only do they develop brighter, more vivid coloration but display amazing natural behaviors that would otherwise not be seen in captivity.
In nature, these fish live in similar environments as Bettas, thriving in cozy thickly vegetated waters. They thrive in densely planted aquariums which offer ample hiding spots and reduce stress. Their presence in a planted tank adds not only aesthetic value but also plays a crucial role in the tank's ecological balance. They help control algae growth through their foraging and their waste is beneficial for plant growth, further integrating them into the planted tank ecosystem. Their small biomass has little effect on water quality, especially in a planted aquarium, and given the proper environment, can thrive and even spawn in very small tanks.
The synergy between these small cyprinids and planted aquariums cannot be overstated. They contribute significantly to the appeal of small planted tanks, encouraging aquarists to explore creative aquascaping possibilities within whatever space is available. By mimicking their natural habitats, aquarists can create vibrant, healthy ecosystems that highlight the beauty and intricate behaviors of these diminutive fish.
Phylogeny and Biological Information
Taxonomic Classification and Phylogeny
The Celestial Pearl Danio (Danio margaritatus), Emerald Dwarf Rasbora (Danio erythromicron), and the various Micro Rasboras belong to the family Cyprinidae, one of the most diverse families of freshwater fish.
The Celestial Pearl Danio and the Emerald Dwarf Rasbora are both Danios and share a close evolutionary history. Markings aside, they are very closely related to each other and similar to other Danios in appearance and behavior. Their brilliant markings, from the star-like speckles of the CPD to the emerald hues of the Emerald Dwarf Rasbora have evolved to blend into heavily vegetated microhabitats. Like all Danios, these fish are very active and tend to spend more time near the surface.
True Micro Rasboras, under the genus Boraras and others, are noted for their smaller sizes, deeper bodies, and typical “rasbora” coloration. They thrive in similar dense vegetation and shallow waters as their slightly chunkier Danio counterparts. They are also masters of the microhabitat. Like all rasboras, they prefer the lower parts of an aquarium.
Adult Sizes and Distinguishing Features
Celestial Pearl Danio: These fish, reaching about 2.5 cm (1 inch) in adulthood, are ideal for nano aquariums. Their notable deep blue bodies with white speckles and vibrant red fins make them a dynamic addition to planted tanks, embodying the spirit of a starry night.
Emerald Dwarf Rasbora (Danio erythromicron): Similar in size to the CPD, the Emerald Dwarf Rasbora displays a striking emerald green and deep blue coloration with vertical bars, they look right at home among dense vegetation, greatly enhancing the aesthetic appeal of planted aquariums.
Micro Rasboras: The most popular species like Boraras brigittae (Chili Rasbora), Boraras urophthalmoides (Exclamation Point Rasbora), and Boraras maculatus (Dwarf Spotted Rasbora) seldom exceed 2 cm (0.8 inches). Each species brings unique hues to the aquatic canvas, from fiery reds to muted oranges, embodying subtlety and elegance in movement and appearance. They tend to have a translucent body with bright spots, likes or specks of vivid color and move with quick tail flips like most rasboras - instead of the side-to-side wiggling motion of Danios.
Discovery and History
Celestial Pearl Danio: Unveiling a Gem
The discovery of the Celestial Pearl Danio (Danio margaritatus) in 2006 is a fascinating tale of chance, keen observation, and rapid popularization. Originally found in small, vegetated ponds in Myanmar by local collectors, it caught the eye of Kamphol Udomritthiruj (a Burmese aquarist) who first brought this species to the attention of the aquarium trade after seeing some for sale in a remote village. The vibrant colors and compact size of the fish immediately captured the imaginations of aquarists around the world. Initially known among enthusiasts as "Galaxy Rasboras," a nod to their star-like spots against a dark blue background (and perhaps a reference to the known Emerald Dwarf Rasbora), their discovery coincided with a surge of interest in planted aquariums where they thrive.
The rapid popularity of Celestial Pearl Danios, was positive and negative; initially, there were concerns about overharvesting in their native habitats. However, successful breeding programs were developed just months after they had been discovered. This alleviated pressure on wild populations, making them more widely available and well-known.
Their rapid time to market after the initial discovery shows just how popular planted aquariums had become as they do not do well in regular fish tanks.
Emerald Dwarf Rasbora: A Close Relative
Similarly captivating, the Emerald Dwarf Rasbora (Danio erythromicron), a close relative of the Celestial Pearl Danio, was discovered in 1918 on the Inle Lake of Myanmar. Although known to science far before the Celestial Pearl Danio, it wasn't until the latter's rise to fame that the Emerald Dwarf Rasbora regained significant attention within the aquarium community. Because its common name as a Rasbora was almost a century old, it remains today. Their sudden popularity after having been forgotten for a century underscores the interconnectedness of these species with the burgeoning trend of planted tanks.
Micro Rasboras: Emerging from Obscurity
Micro Rasboras, including species like Boraras brigittae (Chili Rasbora) and Boraras urophthalmoides (Exclamation Point Rasbora), have a less dramatic history. Originating from various parts of Southeast Asia, these species were gradually introduced to the aquarium hobby over several decades, often by exploratory expeditions in remote regions. The Chili Rasbora, for example, was first described in the early 1990s, but it wasn't until the early 2000s that they became a staple in the hobby, appreciated for their vivid colors and suitability for smaller planted tanks. They never really took off before, perhaps because they lose their coloration, become extremely shy, and often succumb to stress in traditional fish-centric aquariums.
Unlike the singular event that brought the Celestial Pearl Danio into the limelight, the introduction of Micro Rasboras to the aquascaping community was a gradual process. As the trend towards smaller, more detailed aquascapes grew, these diminutive species found their niche. They offered the perfect balance of vibrant color and minimal size, allowing aquascapers to create densely planted, functioning habitats with thriving animals, even in small tanks.
Popularization and Impact on Planted Aquariums
The increasing interest in Celestial Pearl Danios, Emerald Dwarf Rasboras, and the Micro Rasboras has progressed alongside the growing appeal of planted aquariums, a trend that has changed freshwater aquariums forever.
Propelled largely by Takashi Amano’s innovative work on “Nature Aquariums”. Planted aquariums changed the traditional aquarium paradigm by shifting the focus toward the needs of aquatic plants. Amano's use of rimless aquariums, aqua soils, high intensity lighting, and universal CO2 injection, allowed him to make the most of his meticulous aquascaping techniques to create aquariums that had been unimaginable before. For this first time, aquariums were conceived of as living art, rather than prisons for fish.
Amano’s jaw-dropping work initiated a global shift towards densely planted tanks, inspiring enthusiasts to try their own hand at plant aquascaping for the first time. When provided with good substrate, lighting, and CO2, aquatic plants thrived like never before and people soon found their aquariums turning into submerged, lush jungles, allowing for infinite creativity with an ever-growing palette of plants.
Within the rapidly growing aquascaping community, Celestial Pearl Danios, Emerald Dwarf Rasboras, and Micro Rasboras carved out their niche. Their small stature, low bioload, and vivid hues made them ideal inhabitants for compact, lushly planted environments, adding fascinating vibrancy and movement to nano planted aquariums that previously lacked many suitable fish. The increasing popularity of these species created a positive feedback loop that further popularized planted aquariums - especially small ones.
Planted Aquariums Expand Space
Aquarium plants possess the ability to enlarge the perception of space within a given tank. The thoughtful integration of plants significantly broadens the diversity of available habitats, offering ample exploration and interaction zones for suitable fish and shrimp. Adept arrangement of flora can effectively segment the aquarium, establishing essential refuges and territories that prove invaluable for the well-being of fascinating but diminutive and wary species.
The effect of plants is very similar to carving up a big empty space with walls and furniture for human inhabitants. A large, cold, exposed space like a school gym can be converted into apartments that house dozens of people peacefully and happily. Aquascaping not only elevates the visual appeal of the tank to its owner but also cultivates a safer, more comfortable, and engaging environment for its inhabitants. Fantastic additions to any size planted tank, these small, colorful Cyprinids are among the best options for long-term, sustainably happy inhabitants specifically for aquariums smaller than 20 gallons.
The coincidental emergence of these small, enchanting fish species alongside the rise of plant-focused aquariums has profoundly enriched the aquascaping hobby. It has democratized aquascaping as an art, empowering hobbyists to create intricate, lively underwater worlds even when they do not have the space for a huge tank. It’s given more urban residents a chance to create jaw-dropping aquariums in whatever space they can spare.
Because fish (or other animals) are a vital part of any aquarium, these tiny fish have changed the entire planted aquarium hobby, allowing enthusiasts to observe and explore the intricate symbiosis between aquatic animal life and life-giving vegetation - in a way that compliments rather than overwhelms the space they’re in.
Care and Maintenance
Both Micro Danios and Micro Rasboras thrive in environments that closely mimic their natural habitats—soft, slightly acidic waters with dense vegetation. These conditions naturally form in the majority of well-maintained planted aquariums, especially those utilizing aquasoils with CO2 fertilization. This section delves into the specifics of maintaining ideal water chemistry and the specific care these species require to flourish.
Ideal Water Chemistry Parameters:
pH: Celestial Pearl Danios, Emerald Dwarf Rasboras, and Micro Rasboras evolved in very similar habitats in Southeast Asia and share preferred water chemistry including a pH range of 6.0 to 7.5. They prefer slightly acidic conditions but are versatile enough to thrive even in slightly alkaline conditions.
Water Chemistry: Mineral hardness (GH) in their natural environments is low (soft), ranging from 2 to 10 dGH but they are adaptable and thrive in most tap water that is suitable for growing aquatic plants. They do not need RO water by any means and natural processes in planted aquariums tend to lower water hardness (both GH and KH) naturally.
Temperature: These species are comfortable in ranges typical for tropical fish, their natural habitats ranging from 73 to 79°F (23 to 26°C). They are from the subtropics and do well in cooler temperatures than many other tropical species demand.
Tank Setup and Size:
These fish could not be more perfect for densely planted tanks where they are hardy and adaptable. They are much more finicky in comparatively sparse fish “community tanks” of any size as the constant stress of having nowhere to hide from much larger tankmates wears them down, making them more prone to diseases and shock.
Given their diminutive size, Celestial Pearl Danios, Emerald Dwarf Rasboras, and Micro Rasboras are well-suited for small aquariums as long as they are heavily planted. While larger tanks are better for many reasons and will allow you to keep larger schools in a community with other fish, they can be quite comfortable in just 10 gallons - adequate for a small school.
Like most small Cyprinids, they are shoaling fish and need to be kept in groups to be comfortable so you must consider the needs of at least six individuals when considering your aquarium. Luckily they have extremely small bioloads that are easily supported by plants alone. A single Zebra Danio is nearly the mass of 8 Celestial Pearl Danios.
I’ve kept a small group of CPDs in a 5-gallon tank (by themselves) and they thrived enough to spawn. They flourish in setups of all sizes as long as there are densely planted areas. Lush vegetation offers ample hiding spots and territories to explore, which can significantly reduce stress and enable their fascinating natural behavior. The addition of driftwood and leaf litter can enhance aesthetic appeal and also mimicking “blackwater” conditions found in their natural habitats. That being said, “blackwater” is by no means required.
Filtration and Water Movement:
Gentle filtration is recommended to maintain water quality without creating strong currents that could stress these small fish. In larger aquariums, consider lily pipes or even poppy pipes to create gentler movement while providing sufficient water circulation to keep the tank healthy. In smaller tanks, hang-on-back filters with adjustable flow rates are sufficient. These fish have tiny bioloads that will mostly be taken care of by the plants anyways so focus on getting the flow rate perfect to keep water cycling in all parts of the aquarium while avoiding excessive turbulence.
Lighting:
These fish prefer shade especially when resting since they originate from regions abundant in dense vegetation. To mimic their natural habitat in your aquarium, cultivate heavily planted areas which naturally create shady retreats. Good planted aquarium lighting is recommended to grow the lush vegetation they love. They also shimmer almost magically under direct lighting when swimming out in the open.
Behavior and Social Dynamics
Don't be fooled by their tiny size, Celestial Pearl Danios, Emerald Dwarf Rasboras, and Micro Rasboras exhibit fascinating social behaviors and dynamics in the shelter of vegetation. Their inherently shy nature is offset by the security provided by lush plantings, transforming them into bolder, more active members of the aquatic community that can be uniquely fascinating to watch.
Schooling Nature and Behavior in Planted Aquariums
Like many small cyprinids, CPDs and Micro Rasboras are schooling fish with an intrinsic need to be part of a group. This behavior significantly influences their comfort and activity levels. In the wild, schooling is a defense mechanism, and this instinct persists in the home aquarium. A group of six or more individuals is recommended to allow these fish to feel most at ease. In a planted aquarium, these schools are stunning - weaving through the foliage, darting in and out of leaves in a mesmerizing display of coordinated movement.
Because they do not feel as defensive in a densely planted environment, they will eventually break off as individuals or pairs to explore, graze, and even spawn. When threatened, they will quickly school again which you'll notice everytime you have to stick your hand into the aquarium for maintenance.
Peaceful Nature
Their extremely peaceful disposition makes Celestial Pearl Danios, Emerald Dwarf Rasboras, and Micro Rasboras ideal candidates for planted community tanks. They coexist well with other similarly sized and tempered species so long as they have safe spaces. Their non-aggressive nature ensures they are not a threat to tank mates, and given an abundance of cover, they are not easily bullied by other fish.
Finding a Home Among Plants
A planted aquarium provides the cozy environment that enable Celestial Pearl Danios, Emerald Dwarf Rasboras, and Micro Rasboras to exhibit a range of natural behaviors often unseen in captivity. Having plenty of foliage to hide in close proximity makes these fish very active and sociable. Furthermore, the micro-environment created by live plants, including variable light conditions and microfauna, provides these fish with a stimulating habitat that promotes natural foraging and hunting behaviors. They exemplify the interconnected ethos of planted tanks where fish and shrimp are not just inhabitants but integral components of a thriving ecosystem where the interaction between living things weave together a whole greater than the sum of its parts.
Breeding Behavior and Spawning Triggers:
Perhaps one reason why many of these species only became popular with the rise of planted aquariums is the ease with which they will spawn in lush vegetation. Once comfortable among the thickets, CPDs and Emerald Dwarf Rasboras may exhibit specific courtship behaviors that, once noticed, can be encouraged toward spawning with some subtle environmental manipulation. A distinct increase in color vibrancy in males can signal readiness to breed, while females might appear fuller when laden with eggs. Micro Rasboras display similar readiness through more subtle physical and behavioral changes, such as plump bellies and increased activity, exploration, and increased attention toward other individuals.
Since their reproduction is synced with seasonal changes, water conditions are important to trigger spawning. A slight drop in temperature, mimicking the onset of the rainy season in their native habitats, coupled with an increase in live or high-quality foods, can stimulate breeding. Regular water changes can mimic the clean, oxygen-rich rain that falls on during their natural breeding season.
Care of Fry
Post-spawning care is critical for the survival of their very small fry. All these species scatter their eggs among plant leaves or on the substrate, preferring fine-leaved plants or mosses for this purpose. The eggs, being adhesive, stick to plant leaves or fall into crevices in the ground. In a well-planted tank, these eggs are naturally sheltered from adult fish, increasing their chances of survival.
To further protect the fry, breeders can either remove the eggs to a separate rearing tank or, if the main tank is sufficiently planted, leave them to hatch in situ, relying on natural plant cover for protection. The latter approach requires careful management to ensure adult fish do not consume the fry, often necessitating an even more densely planted refuge area within the aquarium and the removal of other, larger species.
Tips for Encouraging Breeding in Home Aquariums
Create a Conducive Environment: Ensure your aquarium closely mimics the natural conditions of these fish, with plenty of live plants, soft, slightly acidic water, and a stable, warm temperature range.
Provide Hiding Places: Dense plantings, especially with fine-leaved plants or mosses, not only provide spawning sites but also hiding spots for fry.
Optimize Water Conditions: Regular, small water changes with slightly cooler water can mimic the natural triggers for spawning.
Feed High-Quality Foods: Increase the frequency of feeding and include live or frozen foods such as brine shrimp or fairy shrimp to condition the fish for breeding.
Consider Tank Mates Carefully: If breeding is the goal, avoid larger or predatory fish that might eat the eggs or fry. A species-specific tank would be ideal.
Breeding Celestial Pearl Danios, Emerald Dwarf Rasboras, and Micro Rasboras in your home is a rewarding experience that underscores the symbiotic relationship between these fish and their vegetated habitats. A well-planted aquarium not only meets their biological needs but also provides psychological willingness to reproduce and an amazing opportunity to observe the fascinating life cycle of these captivating species.
Feeding
Celestial Pearl Danios, Emerald Dwarf Rasboras, and Micro Rasboras can be picky eaters but have become more accustomed to dry food as more are bred and raised in captivity - so long as it fits in their mouths. A varied diet is still crucial to meet their nutritional needs, enhance their vibrant hues, and promote spawning so high-quality food is recommended as are occasional live treats. Planted aquariums naturally house a variety of small organisms like copepods and daphnia that complement regular feedings.
Dietary Preferences and Strategies:
In the wild, all of these fish feed on small invertebrates and zooplankton. In an aquarium, they can thrive on a diet of small-sized foods, such as finely ground flake, micro pellets, and frozen or live foods including daphnia, brine shrimp, and bloodworms. Consider fairy shrimp, which are similar to brine shrimp, live in freshwater, and are found alongside many of these fish in the wild.
In densely planted aquariums, these fish benefit significantly from the natural microfauna that develops. Aquatic plants not only provide shelter for fish and shrimp but food and habitats for small invertebrates like copepods - which you may have noticed in planted tanks without fish. These naturally occurring food sources supplement their diet, providing essential nutrients and mimicking the natural environments. This aspect of planted tanks is especially beneficial for these tiny fish - offering a continuous supply of highly nutritious live foods that bring out their fascinating foraging behaviors. A small planted tank will not spawn enough to feed then alone so supplemental food is still required.
Importance of a Varied Diet
Like with all fish, a varied diet is crucial for maintaining the health and enhancing the coloration of these fish, and may be needed to stimulate breeding. Incorporating a mix of dry and live foods ensures a balanced intake of proteins, vitamins, and minerals along with some variety in their diet. Live foods, in particular, can stimulate fascinating natural feeding behavior. They are often higher in nutritional value.
Suitable Tank Mates
Selecting tank mates for Celestial Pearl Danios, Emerald Dwarf Rasboras, and Micro Rasboras in a densely planted aquarium requires careful consideration because of their small size. Their peaceful demeanor and small stature necessitate companions of similar nature that flourish in comparable, densely vegetated, conditions. A variety of fish with different apperances and behaviors truly reinforce the serene beauty of the planted aquascape - no matter the tank size. Larger aquariums accommodate a wider range of compatible tankmates simply because there is more space but care can be taken to select a variety of suitable species with different niches in even a small planted tank.
Suggestions for Compatible Species
Ideal tank mates include non-aggressive fish and invertebrates that thrive in soft, slightly acidic conditions. Dwarf shrimp, such as Cherry and Amano shrimp, and snails like Nerites or Mystery snails, are excellent choices that contribute to the tank’s ecological complexity and cleanliness.
Other suitable fish companions include Ember Tetras, Pygmy Corydoras, and similar gentle rasboras. These species blend seamlessly into the planted tank's dynamics, coexisting peacefully and adding to the visual appeal of even a small aquarium.
The table below summarizes some species well suited to "nano" planted aquariums and coexist well with tiny Danios and Rasboras. They are all fairly small and prefer a habitat full of plants.
Fish | Scientific Name | Adult Size | Minimum Tank Size | Description |
Neon Tetra | Paracheirodon innesi | 1.5 inches | 10 gallons | Small, peaceful, and vibrant, they thrive in similar water conditions, adding a splash of color. |
Ember Tetra | Hyphessobrycon amandae | 0.8 inches | 5 gallons | Their fiery orange color contrasts beautifully, preferring similar densely planted setups. |
Harlequin Rasbora | Trigonostigma heteromorpha | 2 inches | 10 gallons | Enjoys soft, acidic water, a great companion for a harmonious planted tank. |
Pygmy Corydoras | Corydoras pygmaeus | 1 inch | 10 gallons | Peaceful bottom dwellers that help keep the substrate clean without disturbing plants. |
Otocinclus Catfish | Otocinclus sp. | 2 inches | 10 gallons | Algae-eating habits make them peaceful and suitable tank mates in similar conditions. |
Dwarf Pencilfish | Nannostomus marginatus | 1.5 inches | 10 gallons | Prefers soft, acidic water, complementing the tank's mid-water swimmers. |
Borneo Sucker | Gastromyzon sp. | 2-3 inches | 20 gallons | Keeps algae in check, thriving in well-planted tanks with good water flow. |
Endler's Livebearer | Poecilia wingei | 1.8 inches | 10 gallons | Adds dynamic visual interest without competing aggressively for food. |
Sparkling Gourami | Trichopsis pumila | 1.5 inches | 10 gallons | Appreciates dense plants, adding charm with their iridescence and vocalizations. |
Tips for Avoiding Common Pitfalls in Stocking Nano Planted Aquariums
Refer to the table below for some things to keep in mind when stocking small planted aquariums with fish.
Consideration | Description |
Mindful Stocking | Avoid overcrowding and ensure ample space for all inhabitants to prevent stress and water quality issues. Fish need their own space. |
Behavioral Needs | Some species may require specific swimming space or have territorial needs; understanding each species’ requirements ensures a harmonious tank environment. |
Consistent Water Parameters | Opt for species that require similar water conditions to simplify maintenance and ensure the health of all tank inhabitants. |
Feeding Considerations | Ensure accessible and appropriately sized food for all species while avoiding overfeeding, particularly important in a densely planted setup where natural food sources like copepods can supplement their diet. |
Overall Impact | Incorporating Celestial Pearl Danios and Micro Rasboras into a community planted tank not only showcases their natural behaviors but also emphasizes the interdependence within these ecosystems. These small fish enrich the visual and ecological tapestry. |
In Conclusion
The rise of the Celestial Pearl Danio, the Emerald Dwarf Rasbora and the tiny Micro Rasboras illustrates not only the profound changes that planted aquariums have had on the industry as a whole, but also shifting trends and tastes that have emerged as a result. A fish tank is an awfully unnatural habitat for most of its inhabitants because it is an artificial environment created to keep those animals alive, isolated from their natural environments.
By contast, planted aquariums, seek to create as complete of a natural environment as possible where plants and hardscape do not only delight human viewers but provide a true home for its inhabitants. Even the biggest fish tank is a cage while a much smaller planted aquarium seeks to be a true habitat in which its inhabitants can live their best lives without feeling uncomfortably enclosed.
These tiny danios and rasboras, along other small fish and shrimp have made that goal so much more attainable. We enthusiastically encourage you to give these little guys a try and see for yourself how transformative the experience of owning a planted aquarium can be, not just aethetically, but also for a greater understanding of how disparate elements in nature come together and birth entire ecosystems.