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Keeping and Breeding Rasboras

18 January 2020


The fish we call rasboras are small, colourful schooling fish from East Africa and a wide area of Asia that spans the Indian subcontinent, Southeast Asia, China and Japan. Most of the species aquarists keep come from Southeast Asia where they inhabit a variety of still and gently flowing water habitats. Many species live in streams and ponds, but some live exclusively in peaty swamps, including the most famous rasbora of them all, the Harlequin Rasbora (Trigonostigma heteromorpha).

Although none are truly happy in hard, alkaline water, most rasboras are fairly adaptable and easy to keep provided they are kept in an aquarium with soft to moderately hard, slightly acidic to neutral water chemistry. They are very peaceful and none of them exhibit the nippiness or bullying tendencies seen among some of the barbs and tetras. This makes them especially good choices for use alongside slow-moving fish with long fins, such as gouramis and angelfish.

In recent years some of the very small rasbora species have become popular for use in ‘nano’ tanks down to 45 litres (10 Imperial gallons) in size. These little rasboras, usually Boraras and Microrasbora species, are somewhat delicate initially, but once settled in can do extremely well. They are among the best schooling fish for small tanks, and make ideal companions for freshwater shrimps such as Cherry Shrimps and Bumblebee Shrimps.

Basic care

Rasboras are schooling fish that inhabit the upper water level away from the substrate. They do well in tanks with gentle to moderate water current, excellent water quality, and plenty of vegetation. Floating plants can used to easily provide the overhead shade that rasboras appreciate. Because they are gregarious fish they should never be kept singly or in small groups; keep at least six specimens, and ideally many more than that. All rasboras look their best in large groups, and a school of twenty Harlequin Rasboras in a shady, well planted tank is a breathtaking sight.

Water quality needs to be consistently good, and rasboras are not good choices for immature aquaria. Besides zero levels of nitrite and ammonia, nitrate levels should be as low as possible. Again, floating plants are a good tool here, using up nitrate as they grow and thereby providing a useful way to minimise nitrite levels between water changes.

Water temperature should be middling rather than excessively high; 24-26 degrees Celsius suits most species well, though some prefer slightly cooler water, in some cases down to around 18 degrees Celsius, making such species ideal choices for subtropical and low-end tropical tanks alongside Sewellia and Pseudogastromyzon spp. loaches, Corydoras and other fish that enjoy similar conditions.

Water chemistry

Without exception, rasboras do best in soft, slightly acidic to neutral water. Aim for hardness between 3-10 degrees dH, and a pH between 6.5 and 7.0. While wild fish come from places where the water will often be softer and more acidic than this, such conditions are difficult to keep stable under aquarium conditions, so should be avoided unless the aim is breeding.

If your local water is soft, then maintaining rasboras should be simple, and all that is required is to add dechlorinator before the water is used. Regular pH tests are a good idea to ensure that the pH stay stable between water changes, but lightly stocked soft water tanks should be relatively stable provided 25% water changes are performed weekly.

Aquarists in hard water areas will find things more difficult. Mixing hard tap water with RO or rainwater is the easiest way to provide the right conditions. RO water is safest but expensive to produce, while rainwater is free but has the potential to bring airborne pollutants into the aquarium.

Diet

Rasboras are extremely easy to satisfy in this regard, taking a wide range of foods including flake and micro pellets. Wet-frozen and live foods make a useful adjunct to their diet but are usually not essential except when fish are being conditioned prior to breeding.

Social behaviour

Rasboras are schooling fish and should always be kept in large groups. They mix well with other small, gentle fish that have similar preferences, but being quite gentle fish, rasboras shouldn’t be kept alongside bullying or aggressive species. The larger barbs for example, like Tiger Barbs, can be a bit rough on small rasboras. Ideal companions would include things like Kuhli Loaches, Corydoras, Pearl and Moonlight Gouramis, and non-predatory catfish such as Whiptails and Bristlenose Plecs.

Breeding

Some rasboras are fairly easy to breed, while others are considerably more difficult. Among the easier species are the popular Scissortail Rasbora (Rasbora trilineata) as well as Rasbora dorsiocellata, Rasbora gracilis and Rasbora lateristriata. These species spawn readily in the same way as barbs and danios. They simply need an aquarium with soft water, a slightly warmer than normal water temperature, and feathery plants on the substrate within which the fish can deposits their eggs.

Other rasboras require very soft, very acidic water before they will spawn. Examples of difficult to spawn rasboras include the Harlequin Rasbora (Trigonostigma heteromorpha) as well as Trigonostigma espei, Trigonostigma hengeli, Rasbora kalochroma and Rasbora pauciperforata. When breeding these fish, filtering the water through peat also helps simulate the conditions the fish would enjoy in the wild. Aim for tea-coloured water with a pH around 5.5 and a hardness level between 1-3 degrees dH. At pH levels below 6, biological filtration is not reliable and zeolite should be used to remove the ammonia instead.

Females ready for spawning will be obviously rounded. Feeding the adults with live foods for a couple of weeks prior to spawning will help bring females into spawning condition. Pairs can then be moved into a suitable spawning tank and the water temperature slowly raised to 28 degrees Celsius. The tank needs to be in quiet spot and shouldn’t be too brightly lit. Indirect room lighting is fine, but otherwise ensure any overhead lights are shaded through the use of floating plants such as Indian fern. Within seven days the pair should have spawned if they’re going to. The female will be much thinner at this point. Some species scatter their eggs carelessly, but others, including the Harlequin Rasbora, deposit their sticky eggs on the underside of broad-leafed plants such as Cryptocoryne spp. The eggs hatch after about a day, and the fry become free swimming 3-4 days later, at which point tiny live foods such as infusoria will be consumed. Liquid fry food may also be taken. The fry grow quite quickly, and soon move onto baby brine shrimp and then small daphnia.

Rasbora

Several species in this genus are traded on a regular basis. Among the most frequently seen are the Blackline Rasbora (Rasbora borapetensis), Clown Rasbora (Rasbora kalochroma), the Eyespot Rasbora (Rasbora dorsiocellata) and the Scissortail Rasbora (Rasbora trilineata).

Size varies considerably. The Blackline Rasbora is one of the smallest species, reaching a maximum length of about 5 cm, while the Clown Rasbora is quite a chunky fish at a good 10 cm long when mature. The Scissortail Rasbora is even bigger, at up to 15 cm long and consequently a good choice for jumbo fish community tanks alongside non-predatory tankmates like plecs and loaches.

In general Rasbora spp. are not fussy fish and adapt well to any suitably large aquarium with the right water chemistry. They do not like turbulent water currents though, and do best in spacious tanks with a mixture of open areas and more sheltered thickets of vegetation. Some species are noted jumpers, and Rasbora spp. are not good choices for open-topped tanks.

Trigonostigma

Species in this genus come from peaty swamps where water current is minimal. By far the most commonly traded members of this genus are Trigonostigma heteromorpha and Trigonostigma espei. Both may be sold as the Harlequin Rasbora, and they are very easy to confuse. Both are deep-bodied, copper-coloured fish with black L-shaped markings on their flanks. The true Harlequin Rasbora is Trigonostigma heteromorpha though, and the markings on its flanks are more triangular, whereas the equivalent marking on Trigonostigma espei, sometimes called the False Harlequin or the Lambchop Rasbora, is nearer an L in shape than a triangle. A third species is seen far less often, the Glowlight Rasbora Trigonostigma hengeli. Although similar to the other two species, this species has a bright coppery-orange band on top of the black markings on its flanks.

Maintenance of Trigonostigma species is consistent. All species are small (to 5 cm) schooling fish that require soft, acidic water and low light levels. They are hardy given the right conditions, but will not last long in hard, alkaline water. The tank should be thickly planted and any overhead light filtered through a canopy of floating or tall plants. For general maintenance keep them at 24-26 degrees Celsius, though slightly warmer water will be required for spawning.

Boraras and Microrasbora

These miniature species rarely exceed 4 cm in length, and some species are not even half that size. As such have become very popular choices for small planted tanks. Commonly traded species in the genus Boraras include Boraras brigittae, Boraras maculatus, Boraras merah, Boraras micros, and Boraras urophthalmoides. Within the genus Microrasbora, regularly encountered species include Microrasbora erythromicron, Microrasbora kubotai, Microrasbora nana, and Microrasbora rubescens.

At the tiny end of the size range are things like Boraras micros (to 1.3 cm), Boraras merah (to 2 cm), Microrasbora kubotai (to 2 cm), and Microrasbora erythromicron (to 2 cm). In general terms maintenance is similar to Trigonostigma such as Harlequin Rasboras, except that being that much smaller, tankmates need to be chosen more carefully. Ideally schools of these fish would be kept on their own in a planted tank with no other companions except perhaps algae-eating shrimps and snails. Otherwise, only the most gentle aquarium fish would be chosen, such as Dario spp. perch and pygmy catfish like Corydoras hastatus.

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