The Genus Boraras: Miniature Jewels

Author: Mike Hellweg

Miniature fishes have enjoyed a renewed popularity of late, and Mike Hellweg reviews some of the most sought-after for smaller tanks: the cyprinid genus Boraras.

The Aquarium Renaissance

In the past decade, the aquarium hobby has experienced a renaissance of sorts. There has been a massive surge in the popularity of miniature fishes, freshwater invertebrates, and so-called nano tanks. At any pet shop, the choices of miniature aquaria are limited only by the shelf space available. Several manufacturers make many different models of plug-and-play mini tanks, from just a couple gallons up to about 20 gallons. Along with these diminutive tanks, miniature fauna have grown in popularity to animate these small habitats. One of the most colorful and popular groups of fishes in the hobby are the cyprinids, and many miniature representatives of this massive family fit the bill as residents of these nano tanks.

The most popular nano-tank-compatible cyprinids by far are the six or so members of the genus Boraras. None of them exceed an inch, though one can come close. It’s no wonder they have become so popular. They are inexpensive, beautiful, hardy, adaptable, easy to care for and relatively easy to breed when given the right conditions. They also have fascinating and surprisingly complex behavior.

As with any relatively new group of fishes, confusion and misinformation regarding this genus abounds in the literature and on the internet. I hope this article, based on my experience working at one time or another with all of the known species of Boraras over the past 20 or so years, will help alleviate some of this confusion.

Aquarium Care

There are currently six recognized species, and their diminutive sizes are better described in millimeters rather than inches: Boraras maculatus, the dwarf or pygmy rasbora (22 mm); B. micros, the dwarf Thai rasbora (13 mm); B. urophthalmoides, the exclamation point or sparrow rasbora (16 mm); B. merah, the neon orange dwarf rasbora (16 mm); B. brigittae, the mosquito, chili, or red rasbora (18 mm); and the most recently described species, B. naevus, the strawberry rasbora (12 mm), which was also called B. sp. “South Thailand” and B. sp. “red micros” for a while. A couple more possibly appear in the trade from time to time, but they don’t yet even have trade names and usually come in with one of the known species names attached.

Tank Requirements

While they have become popular fish for nano tanks and do well in such tanks, they actually are at their best in larger single-species groups in well-planted tanks like 15- or 20-gallon longs. My favorite tank for a group of two or even three dozen is a 20-gallon long with dark gravel, thickets of Cryptocoryne and Anubias, a few floating plants to help reduce the light, and a couple of pieces of real driftwood or a pile of oak leaves on the bottom. The presence of driftwood and/or oak or other hardwood leaves becomes of key importance in breeding, so it is, in my opinion, necessary for a Boraras tank.


In the wild, they are most frequently found in backwater microhabitats with shallow, slow-flowing water, so they do best with slow-flowing water in our tanks, too. I use and highly recommend mattenfilters (see my 2010 blog entry on Mattenfilters for more info) that take up one of the short sides of the tank. This provides a slight bit of current near the surface and a slow, even flow throughout the tank. My mattenfilters are covered with Java moss, Anubias, and Java fern, which makes them even more useful as biological filters and water purifiers. Any type of sponge filter will work, as will good old undergravel filters if properly maintained. Small canister filters and small inside-the-tank power filters also provide good filtration without too much current.

Water Requirements

Water parameters are important if you are starting out with wild-caught fish. Most of the miniature cyprinids come from blackwater areas where the pH can be as low as 4.0, with no measurable hardness at certain times of the year. There is a reasonable argument that miniaturization in many of these species is actually a response to living in these harsh habitats where minerals for bone growth are scarce.

Wild-caught Boraras should first be kept in water that is as close to their wild conditions as possible. With wild-caught fish, I first add a commercial peat extract to the water at double or even triple the dose, and I use about 50% reverse-osmosis (RO) water. I’ve found they are very adaptable and within a few months, with small, regular water changes using gradually less and less RO and peat extract, they can adapt very readily to harder, more alkaline water, though breeding in some species may be much more difficult to achieve in such water. I’ve kept and successfully bred all six species of Boraras in my local tap water, which has a varying pH of 6.8–7.2, with a total hardness of about 120 ppm and a carbonate hardness that varies from near zero to about 60 ppm depending on the time of year.

Foods and Feeding

Boraras are micropredators, so they need meat in their diet. They will take finely ground flake, powder, and micro-pellet foods, as well as frozen foods like copepods, brine shrimp nauplii, and similar fare. But to condition them for spawning, there is nothing like live foods. Mine get a varied live diet, with a carnivore type flake or pellet added as a supplement a couple of times a week. They take live brine shrimp nauplii, small daphnia, moina, copepods, Grindal worms, and similar-sized items.

The one thing I’ve found that really helps ramp up spawning is feeding them the small nematodes hobbyists collectively call microworms. Whether going by the moniker microworms, potato worms, banana worms, or even Walter worms, it doesn’t seem to matter. I’ve tried them all, and they all work. The key is feeding not only the worms that climb the sides of the culture container, which are mostly adult worms, but also dipping into the media itself and getting the smaller, younger worms, upon which even newly free-swimming fry can feed.

The Secret Ingredient

The other key to successful Boraras husbandry in my experience is having either decaying wood or leaves in the tank. When you think about it, it is one of those head-slapping, “well, duh” moments. Their native blackwater habitats are full of fallen branches, tree trunks, and leaves. In some places, the fallen leaves are so thick that the water isn’t even visible. In others, it appears to be just a few inches deep, even though it may be several feet deep! As the wood and leaves slowly decay in the water, they become home to some unique microflora and microfauna that doesn’t grow anywhere else. They release into the water tannins and other products that lower the hardness and the pH, eventually creating the tea- to coffee-colored water we hobbyists know as blackwater.

As far as food goes, the one big thing about this water is that it is fairly sterile, with little microscopic life at the bottom of the food chain. I believe Boraras fry (and those of other small, blackwater cyprinids) have adapted to this challenging environment by taking advantage of another, very abundant food source. I’m not sure whether they feed on bacteria, fungi, or some other microscopic life that feeds on the leaves or driftwood or even something that eats the things that eat or break down the leaves and driftwood, but that slick, slimy coating on the driftwood and leaves appears to be important to their early feeding strategy.

I first noticed this back in 2003 when my B. urophthalmoides group started reproducing. I kept them in a 20 high that was thickly planted with Cryptocoryne and Anubias growing on a large, branchy piece of driftwood. The fry, even when barely free-swimming, were all hanging around the driftwood. When I separated a few adults to breed them the “normal” way in a separate spawning tank, I would get free-swimming fry but they would never grow, even with heavy feedings of paramecia, infusoria, green water, Cyclops, etc. By chance, I added a piece of slimy driftwood from an established tank to the fry tank. In the mornings the newly free-swimming fry could be seen congregating around it with full bellies, even though I had not yet fed them that day. They also, as mentioned before, eat things like paramecia, infusoria, copepods, and baby microworms, but the “slime” or whatever lives on it seems somehow critical. I’ve tried it with all six species, and it works with them all.


As with any new but popular fish, all kinds of myths appear. With the internet, these myths spread quickly around the world and become “facts.” Boraras are no exception. For a while after their first import back in the 1950s, Boraras (then Rasbora) maculatus were reputed to be the juveniles of another species of Rasbora. It didn’t take long to figure that one out, as the other species, R. kalochroma, grows to three inches while B. maculatus doesn’t quite make it to an inch.

About a decade ago, B. urophthalmoides was widely available as the exclamation point rasbora. A new species, B. brigittae, started coming in, and many hobbyists immediately called it simply a red morph of B. urophthalmoides. But B. urophthalmoides and B. brigittae are found hundreds of miles apart, with the South China Sea separating them. When scientists looked at them closely, they found the two species aren’t even in the same clade.

Finally, the most recent Boraras myth is that B. merah is not a valid species, but in “fact” is the female of B. brigittae! The proof was supposed to be that the two species interbreed freely. This comes from the fact that both B. merah and B. brigittae are closely related and are found on the Island of Borneo, where their distribution overlaps. Both species are often lumped together in the collection bucket (along with possibly another undescribed species) and later wind up being exported in the same shipment under one name. When hobbyists get them at stores, the specimens are still mixed together. These mixed species will interbreed freely and fairly easily, but when known B. merah and B. brigittae are crossed in the aquarium, their progeny are, in my experience, sterile, indicating that they are, indeed, separate species. It may not always be the case, but I’ve tried several times with several different specimens from different non-mixed collections and always had the same results.


All six species of Boraras have strong sexual dimorphism—that is to say that mature males and females are easy to tell apart. In five species, males have bright-red or even neon-orange coloration in the unpaired fins and on the body. In B. micros, males have a bright-lemon-yellow body and clear fins. In all six species, females generally have little to no red, clear fins, and a much fuller profile. They are slightly larger than the males, but in fish that are just over a half-inch long, that isn’t saying much!


In all species, females generally school loosely with juvenile or subdominant males. Dominant males literally glow with color and guard breeding territories. They fight with one another constantly, maintaining their territory with mock battles, in which they swim alongside each other head-to-head or circle each other head-to-tail, swishing and flaring their fins, testing each other’s strength, and occasionally nipping each other’s fins. They also display for the females, doing a little dance in front of the group of females, quivering and possibly releasing pheromones into the water to let the females know they are ready to breed. Sometimes they drive females from the group into the plants in their territory, but mostly the female seems to leave the group and willingly follow the male into a thicket of plants.


Spawning is fairly quick, and if you blink you’ll miss it. They do a side-by-side quiver, sometimes a barrel roll, sometimes not, and often lay their eggs on the underside of plant leaves, though just as often, they lay their eggs in the plants. I’ve had many successful spawns (B. maculatus, B. micros, B. naevus) with nothing more than Java moss on a piece of driftwood in the tank. Generally, the females lay just a single egg or maybe two at a time and can lay a half dozen to a dozen a day over a week or so. Males appear to always be ready to spawn. They display, court, and fight every day and pay no attention to their progeny after spawning.


In a species tank set up as described earlier, with the driftwood, plants, leaves, and daily feedings of live foods, especially microworms, fry appear regularly with no other work on the part of the aquarist. The adults don’t seem to consider the fry as food, so they ignore them. One question I often have asked when I’m giving a talk about miniature cyprinids is whether or not it’s okay to keep shrimp in the tank. If you just want a nice display tank, shrimp are fine. If you want to build up a colony of Boraras, I would leave shrimp, snails, and other fish out of the mix. Even if they don’t eat the eggs, they will compete with the Boraras for food and interrupt spawning attempts.

Another interesting phenomenon that I’ve noticed, not only with most of the species of Boraras (all but B. maculatus) and many other miniature cyprinids, such as Danio margaritatus, D. choprae, D. nigrofasciatus, Danionella dracula, Paedocypris spp., Horadandia atukorali, and Sundadanio sp. red/blue), is that if you remove the adults and leave the young to grow out in the tank, when the first group reaches about 10 weeks of age, they start breeding and you wind up with a large number of tiny fry appearing in the grow-out tank.

Great Fish in a Little Package

If you’re thinking of a nano tank and want bright color, a load of individuality, and ease of care, you can’t go wrong choosing one of the diminutive species of Boraras for your tank. If you set up a larger species tank, you’ll be rewarded with a long-lived miniature fish with lots of behavior that is surprising coming from such small fish, as well as the possibility of an ever-growing colony. What more could you ask of a fish less than an inch long?

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