Rasboras: The Spartacus fish


What is the true rasbora? Just as in the film epic many will make a claim and create even more confusion. Nathan Hill investigates.

Say hello to one of those complicated groups of fish where historically there seem to be more arguments over names than there are actual species.

Rasbora, at least those species commonly called rasboras, seem victims of a taxonomic gold rush, where researchers rush east, jam flags into species old and new, then ruminate over their exact biological origins.

Newer fishkeepers tend to be greeted by names that conflict from store to store. Older fishkeepers will fondly recall species that used to be Rasbora now belonging under some other heading.

Technically, one could quibble that only those fish within the Rasbora genus could be classed as such. However, some of us prefer a broader definition that sees numerous genera all carrying the title, so I’ll take the stance of many keepers in classing fish from several genera under the honorary title including, but by no means limited to, fish such as Trigonostigma, Boraras, and Rasboroides all considered to be rasborin fish.

Much of the confusion abounds simply because the name Rasbora is an Eastern Indian word that describes a fish.

It also means that there are many fish trading as Rasbora that do not make the grade, even though common names suggest otherwise. Examples of 'fraudboras' include the Asian rummynose (Sawbwa resplendens), the Galaxy rasbora (Danio margaritatus) as well as Microrasbora species.

Rasboras are cyprinids, that huge group carrying everything from huge Koi carp to dinky Paedocypris, and much in between. Characteristically, Rasboras are noted for not having any of the distinctive barbels associated with cyprinid fishes.

Tank delights (Picture above by George Farmer)

Rasboras, true or otherwise, are superb aquarium inhabitants and lend themselves to some stunning biotope set-ups.

Their typically small size, combined with only limited shyness and shoaling behaviour, makes them stunning additions to many communities, although their water chemistry requirements sometimes prevent them being kept this way.

It’s not uncommon to see shoals of Harlequin rasbora (Trigonostigma heteromorpha) used to devastating effect among thick beds of dense green foliage in the aquarium. Aside the often gin-clear tint of aquascape water, such tanks aren’t far off being ideal for these fish.

The tiny Boraras species, as well as bearing spectacular colours, require only small systems in which to thrive and are dominating nano tanks and micro-biotopes these days, despite the occasional high price for such a small fish. But when you see one of these close up you’ll be hooked — and you’ll soon be dusting off that old 45 x 25 x 25cm/18 x 10 x 10” tank.

Different species frequent different tank levels, many capitalising on the mid-water layer.

All are easy to feed, with every species adapting to flake foods and life in captivity superbly. They retain their ravenous instincts, despite aquarium life, and dashes of live food always create frenzied activity in the tank at mealtimes.

Anything goes, but Daphnia, small bloodworms, glassworms, wingless fruit flies, and even insects such as garden ants will never be refused by these fish.

Which set-up is best for you?

Harleqin rasbora

The Harlequin (Trigonostigma heteromorpha) has been a long-standing favourite in the trade and the rasbora most fishkeepers are likely to keep. They tolerate a wide range of water conditions, are attractive and active.

It’s thought there are two species of this fish, depending on the presence of an orange spot on the anal fin, but whichever Harlequin you pick up will be rewarding.

Wild Harlequins are a scarce find nowadays, and the vigilant or purist might want to avoid colourful morphs, such as the blacks and blues you can see around. Good Harlequins are available in the shops but, like so many mass-farmed fish, there can be runts.

These fish go well in a community, but to see them at their best, try to create a biotope, or perhaps a species-only tank.

Found across Indonesia, Sumatra and Thailand, they usually live in acidic but not deeply blackened streams, and sometimes peaty swamps. Those streams tend to have a yellowed tinge, and this is something the home aquarist would want to replicate if possible.

Decoration is easy and ideally there should be plenty of it. Ample pieces of bogwood should litter a sandy base, and that sand should be fine and dark, such as aquarium-grade silica.

Leaf litter can be added and will help to release tannins into the water. Alder cones may also be used, but be sparing as they can be quite powerful in releasing acid.

These fish prefer softer, acidic waters and although they can cope in a neutral or slightly alkaline community tank, it’s worth remembering that some pathogens fare better in alkaline waters. Aim for a pH of between 5.0 and 7.0, with a hardness of below 12°DH.

Set the temperature somewhere between 22-25°C/72-77°F to recreate wild conditions and keep water flow light. Harlequins do not thrive in high, rushing water flows and may actively avoid areas in the tank where they get buffeted.

Planting works well with Harlequins, especially thick beds of Cryptocoryne and Java fern. Don’t be shy about going bonkers with numbers of plants and the Harlequins will love you!

To really promote their confidence, add floating plants to the tank. Even a handful of duckweed will encourage them to be more brazen.

They only grow to 5cm/2” as adults but do enjoy being in shoals, so it’s worth thinking of a tank no less than 60cm/2’ long, with at least 12 Harlequins.

These fish are peaceful with everything, but to keep things region authentic, opt for tank mates like Acanthopthalmus loaches, Puntius species like P. pentazona, or maybe even Chocolate gouramis.

Rate of difficulty: 2/5

Clown rasbora

Rasbora kalochroma is a big boy and needs a larger home. At 10cm/ 4” in total length and, with a habit of displaying between males, this rasbora does much better in a tank of at least 120cm/4’ in length.

Clowns prefer more flow, although the water should still be stained with tannins. To this effect, it’s worth considering adding alder cones to a external canister filter, or some peat supplementation if these aren’t available. Tannins will help to bring out the red colours of the fish, which can be a washed-out orangey-yellow when kept in water that’s too clear.

Unlike with Harlequins, planting should be confined to the backs and sides of the tank and ample swimming space should be left in the middle where males will show off to each other. Cryptocoryne is a good choice, but dense, bushy plants, like thick mats of Java moss, are appreciated. The fish will often be seen ‘rubbing’ themselves in it.

Crucially with Clown rasbora, the tank needs to be covered as they are prone to launching themselves at first fright. Surface cover and floating plants will often help.

Acclimatisation is essential and they are easily shocked as a result of changes in water quality. As imports go, they are riskily high on mortality, which is why they are not as commonly seen as other species.

When adding them to your tank, try to match water parameters to the transport water, but don’t be too surprised if they pale, pant, and rest on the tank base on arrival.

The beauty of the Clown rasbora biotope is that you can have so many more fish in their tank and they are naturally found with a wealth of other species.

Along with other Rasbora species, they will also happily co-exist with the icy-eyed Brevibora dorsiocellata, some Puntius species, Chocolate gourami, and even some Betta, such as B. pugnax.

There’s confusion about numbers to keep. Some maintain that due to their pugnacious attitude towards each other, low numbers are best. However, it’s generally accepted that these are shoalers and should be kept in groups of 12 or more.

In the wild, these fish get everywhere. From streams and rivers, through pools and lakes, and into rice paddies and even ditches on the side of the road, these are a surprisingly versatile fish that should be given the chance to shine.

Aim for a lowish pH between 6.0 and 6.5. A hardness level somewhere about 6°DH seems to suit. Given a temperature of about 25-28°C/77-82°F and access to plenty of live foods and carotenoid rich dry foods, these are fish you’ll quickly come to adore.

Rate of difficulty: 3/5

Mosquito rasbora

If the scientific name Boraras urophthalmoides sounds like an anagram of rasbora, it’s because it is. Boraras represent truly tiny members of the rasbora grouping and are among some of the most shy. However, given their fiercely bright coloration, they have a large fan base. I’d guess that they are one of the UK’s most popular biotope fish.

B. urophthalmoides inhabit truly acidic waters. In fact, they’re the archetypal swamp dweller, relishing blackened water, low light levels and relentless acid.

Although some of them are more slow stream dwellers, the biotope habitat of a peaty swamp is acceptable to the Mosquito rasbora and makes for an interesting and unusual project.

Acidity and blackness should be high. Keepers like to use heavy leaf litter, especially oak and beech leaves, combined with spindly pieces of bogwood, such as the Sumatran driftwood from Unipac, and low lighting levels.

Some keepers don’t even opt for an aquarium light, instead using a small low wattage table lamp. Although some plants like Java fern might tolerate this, it could be wiser to go for a coating of floating plants instead. These fish are from regions of overhanging foliage and natural light is quite alien.

Water chemistry should be completely soft and acidic. A pH value as low as 4.0 will be shrugged off, along with hardness levels of zero to 10°DH. Aim for a pH of between 5.5 and 6.5.

They don’t do well in nitrogenous wastes, so ensure that any tank is established prior to adding the fish.

Water flow should be negligible and a tank of B. urophthalmoides is the perfect excuse to dig out that old Biofoam 45 air-driven filter, and an air pump.

Mosquito rasbora work best as a species-only shoal, due to their timid nature and small size. It’s quite easy for them to end up in the belly of a normally peaceful tank mate.

However, if wanting to keep them with another species, consider the equally timid Parosphromenus varieties of tiny Liquorice gourami. Although you’ll not see too much of either species, you’ll be ecstatic when they do make an appearance through that deep red water.

They struggle to get past 15mm/0.7” in length, which is why Mosquito rasbora are appearing in so many nanos, although if kept in too small a set-up you can expect rivalry between males.

Rate of difficulty: 3/5

A miniature world!

There’s a reason many rasbora and related families are so small — and it’s evolutionary.

The fish have gradually become smaller, it is suspected, through environmental pressures.

All miniaturised fish inhabit slow moving or static bodies of water, often devoid of sources of nutrition, such as the conditions you might find in a boggy, peaty swamp.

This evolutionary niche loses its advantage anywhere that water flow is strong and nutrition widely available.

Usually, a miniaturised fish will be stunted and small, but perfectly formed.

However some fish, like Paedocypris — the world’s smallest — stops short of forming fully adult organs. Instead it retains a simplified, or truncated body in which even the skeleton isn’t fully developed.