The Harlequin Rasbora is a shoaling fish with a copper-orange body and a bold black wedge, a planted-tank classic and one of the best beginner fish.
Overview
Trigonostigma heteromorpha is arguably the best-known rasbora in the hobby and one of the fish that home planted tanks were built around back in the middle of the last century. The species comes from the peat swamp forests and forest streams of Malaysia, Singapore, southern Thailand and Sumatra, where the water is soft, acidic and stained brown with tannins, and the bottom is carpeted in fallen leaves.
Adults reach 4-5 cm and live 5-7 years — noticeably longer than most small cyprinids. The body is deep and solidly built, coloured a warm copper-orange to pinkish apricot, with a large charcoal-black wedge across the rear half that gives the fish its trade name. Its closest relative, Hengel's Rasbora, carries a wedge three times narrower trimmed with an orange stripe, on an almost transparent body.
The shape of that wedge is also how you sex them: in males the lower front corner extends forward into a point, while in females it is straight, rounded and ends noticeably closer to the tail. Males are additionally slimmer and more intensely coloured, females fuller in the belly.
In temperament T. heteromorpha is a completely peaceful, diurnal shoaling fish that holds the middle layers. Keeping it is straightforward, but one mistake is made constantly: harlequins are bought five or six at a time. At that number they scatter and look washed out. A proper tight shoal starts at 10-12 fish — and that is the species' one serious requirement.
Tank Requirements
The minimum is 40 litres for a group of ten, but the comfortable option is a tank of 60-80 litres and 60-80 cm long. Harlequins swim a great deal and willingly, so length matters more than height.
Water parameters: temperature 24-28 °C, pH 6.0-7.5, general hardness (GH) 5-15 dGH. The range is wide, and that is the species' strength — it lives happily in ordinary conditioned tap water of medium hardness. For the best colour, and certainly for spawning, soft slightly acidic water is needed: pH 6.0-6.5, GH 5-8.
The layout is the planted-tank classic: fine dark substrate, dense planting around the perimeter and an open swimming zone in the middle. Do include broad-leaved plants — cryptocorynes, anubias, swords: in nature harlequins spawn underneath exactly these, and in a community tank a pair will stage regular "rehearsals". Driftwood and dried almond leaves acidify the water and lend it a faint amber tint that shows the orange body off at its best. Floating plants soften the light from above — under bright open lighting the fish pales and hugs the bottom.
Filtration should be moderate, without strong flow. A lid is mandatory: the species jumps, especially in the first days after introduction.
Feeding and Diet
In the wild T. heteromorpha feeds on small insects and their larvae, worms and zooplankton. The mouth is small, so food must be fine: micro-pellets, quality flakes or crushed chips.
Two or three times a week add live and frozen foods — daphnia, cyclops, brine shrimp, small bloodworms. This is the main tool for maintaining colour: on dry food alone the copper tone washes out to a pale beige within a couple of months. A vegetable component helps too — a spirulina-based food once a week.
Feed twice a day in portions consumed within a minute. Harlequins take food in mid-water and rarely go down to the bottom, so never add more than the shoal can intercept. Include one fasting day a week: the species runs to obesity, and overfed females stop spawning.
Care and Maintenance
A weekly 20-30% water change is the baseline routine. Prepare the water in advance and match the temperature: abrupt swings during a change are the most common trigger of an ich outbreak in a rasbora tank.
Rinse filter media in old tank water every three to four weeks. Vacuum the substrate in open areas, leaving plant roots alone and not stripping out the leaf litter entirely if you are using it to acidify. Target readings: ammonia and nitrite at zero, nitrate up to 20-30 mg/l.
A healthy shoal is tight, active and evenly coloured, and the fish are first out at feeding time. Cause for concern: the shoal breaking up, hanging in corners, a fading wedge, clamped fins or white spots on the body. The main diseases are the ich already mentioned, bacterial fin erosion where organics run high, and fungal infections in fish weakened by transport. Quarantine new arrivals for two to three weeks.
Longevity deserves a separate word: with stable parameters and moderate feeding harlequins live six to seven years, whereas in an overfed tank with infrequent water changes they manage half that.
Compatibility
Trigonostigma heteromorpha is harmless to every tank mate and fits into practically any peaceful community. The only constraint is not to house it with anything capable of harming it.
Ideal companions: Hengel's Rasbora, Neon Tetra, Ember Tetra, Celestial Pearl Danio, Cherry Barb and Sparkling Gourami. The bottom level is covered by Panda Cory, Pygmy Corydoras, Half-Banded Loach and Dwarf Suckers. For algae control the Siamese Algae Eater works well, and among larger peaceful species the Chocolate Gourami fits if you keep the water soft.
Harlequins coexist with shrimp, but the fish is larger than Hengel's Rasbora and takes newborn shrimplets more readily — do not count on breeding neocaridina in a community tank.
Unsuitable: the Tiger Barb and other fin nippers; boisterous Zebra Danios in a small volume, which intercept the food and keep the shoal on edge; and any cichlid or fish whose mouth accommodates a five-centimetre body. Pairing with a Betta is possible but depends on the individual male's temperament.
Breeding
Breeding T. heteromorpha is harder than keeping it and rates as intermediate — not because of the fish's behaviour but purely because of water chemistry. Above 4 dGH the overwhelming majority of eggs fail to fertilise or go white within the first day, so ordinary tap water simply does not work here.
Set up a 20-30 litre breeding tank: RO-based water at GH 1-3 dGH, pH 5.5-6.5, temperature 27-28 °C, no substrate, dim lighting. The essential element is several broad-leaved cryptocorynes or anubias positioned some 10-15 cm below the surface. Use an air-driven sponge filter only, on minimal flow.
Condition the breeders separately for seven to ten days on generous live foods. Introduce one ripe female and two males in the evening. In the morning the male begins display runs beneath the chosen leaf, after which the pair adopts the characteristic posture: both fish turn belly-up and press against the underside of the leaf, where the female glues a batch of eggs. Across several passes 50-100 eggs are laid. Remove the adults as soon as spawning ends.
Incubation takes 24-36 hours, and the larvae then hang from leaves and glass for another three to five days, using up their yolk sacs. Free-swimming fry are given infusoria and liquid starter food four to five times a day for the first few days, moving on to brine shrimp nauplii and microworms from a week old. Siphon leftover food off the bottom daily and change 10% of the water, raising the hardness very gradually over a month towards community-tank values. The signature black wedge appears on the juveniles at six to eight weeks.