Harlequin Rasbora Trigonostigma heteromorpha

Quick Stats

Tank Setup

The Harlequin Rasbora (Trigonostigma heteromorpha, formerly Rasbora heteromorpha — the species was reclassified from Rasbora to Trigonostigma in 1999 when taxonomists realised it spawns on the undersides of broad-leaved plants rather than scattering eggs in the water column, a behaviour shared with the related species T. espei, T. hengeli, and T. somphongsi) is the classic nano rasbora. Copper body, blue-black triangle on the rear half, schools tight enough to turn as a unit, hardy enough to live in almost any dechlorinated tap water on the planet. It is the fish most of us picture when someone says "schooling nano fish," and the one I steer first-time planted-tank keepers toward when they want a school that looks intentional rather than accidental.

Water parameters: temperature 22–28°C, pH 5.0–7.5, hardness 2–12 dGH. That range covers almost any tap water in any temperate house — harlequins are unusually tolerant for a soft-water Asian species, which is a big part of why they have been the default community-school fish for 60+ years. Captive-bred stock (which is everything in shops) will live and breed at pH 7.2 and GH 10 without complaint; wild-caught stock is harder and benefits from tannin-stained, acidic water. I run my harlequin group at 25°C, pH 6.8, GH 6 — straight tap water, no RO, no active substrate — and a school of 10 looks exactly like the textbook photographs. Avoid sustained temperatures above 30°C, which suppress immunity; below 20°C the fish slows down, stops eating well, and becomes susceptible to ich.

Set up the tank with a dark sandy substrate (black or dark brown shows the copper colour best), driftwood, and dense planting along the back and sides. Floating plants (Salvinia, frogbit) are welcome but not strictly required; harlequins are midwater fish rather than surface fish and spend their time in the middle third of the water column. The non-negotiables for spawning are broad-leaved plants (Anubias, Cryptocoryne, Java fern, Echinodorus) — harlequins spawn on the underside of leaves and will not breed successfully in a tank without them. Filtration should be moderate — a sponge filter or a gentle hang-on-back is fine. Strong current disperses the school and stresses them. The black triangle pattern is the species' signature: a blue-black wedge that starts at the lower rear of the body and tapers backward toward the caudal peduncle, with its rear edge roughly straight and angled toward the bottom of the tail. Males have a slightly more rounded lower edge to the triangle and a deeper copper above it; females are fuller-bodied with a straighter-edged triangle. The pattern is consistent across the species and is the easiest way to identify a true Harlequin versus the slimmer, more lambchop-shaped T. espei (Lambchop Rasbora) or the smaller, orange-finned T. hengeli (Glowlight Rasbora).

Tank Mates

Harlequin Rasboras are peaceful, midwater, and too small to bother anyone. They are the fish that other tank mates are designed around, not the fish that needs designing around. The two questions to ask are: (1) is the tank mate small enough not to eat a 4 cm rasbora, and (2) is the tank mate peaceful enough not to nip the rasbora's fins. If both answers are yes, the pairing works.

Compatible tank mates: every common nano community fish. Harlequins live happily with tetras (Ember, Neon, Cardinal, Green Neon, Rummy-nose, Glolight), other rasboras (Chili, Galaxy, Lambchop, Scissortail — though scissortails get larger and need a 20+ gallon), Corydoras of every species, Otocinclus, kuhli loaches, pencilfish, dwarf gouramis, honey gouramis, and dwarf cichlids (Apistogramma, rams, kribensis). They are excellent dither fish for a shrimp tank — their mouths are too small for adult shrimp and they don't actively hunt the bottom. Snails are completely ignored. A single Betta can work in a 15 gallon with harlequins if the betta is mellow, but harlequins are fast enough to escape a betta's aggression without much stress — they are not long-finned nipping targets the way guppies can be.

Avoid: any fish large enough to eat a 4 cm rasbora (angelfish will try, Oscars will succeed, goldfish are too big and too fast-growing), nippy fish (tiger barbs, serpae tetras, silver dollars — they will shred the rasbora's fins over a week), and aggressive cichlids of any size. The harlequin's wide water tolerance means you can pair them with hard-water livebearers (guppies, platies, mollies) in a community tank — something you cannot do with the smaller, softer-water Boraras rasboras. In a 20 gallon long, a school of 10 harlequins plus a school of 10 Ember Tetras plus a group of 6 Pygmy Corydoras is the textbook nano community — midwater, lower-mid, and bottom all covered, all peaceful, all compatible on water parameters.

Diet & Feeding

Harlequin Rasboras are omnivores. In the wild they eat insects, micro-crustaceans, zooplankton, and soft plant matter — they are midwater feeders that pick food out of the water column rather than surface-feeding or bottom-feeding. In the aquarium they accept every common food: flakes (high-quality tropical flakes, not the cheap stuff), micro pellets (Hikari Micro Pellets, New Life Spectrum Thera+A 0.5 mm, Fluval Bug Bites), frozen daphnia, frozen brine shrimp, frozen cyclops, live baby brine shrimp, and the occasional live blackworm. They are not picky and will be in condition on flake alone, but colour responds fast to variety — a week of frozen daphnia and BBS turns a pale shop-fresh group into a deep copper-and-black textbook school.

Feed once or twice a day, what they can clear in 30–60 seconds. Harlequins are surface-midwater feeders by habit but will chase food to the bottom; they are not as committed bottom-feeders as corydoras, and food that hits the sand and stays there will be ignored. A pinch of flake at the surface, a small pinch of micro pellet mid-water, and a small portion of frozen daphnia twice a week is the routine that gets you the best colour. Avoid overfeeding — harlequins are easy to overfeed and a fat harlequin loses the streamlined shape that makes the species look right. Target the body shape in the textbook photos: torpedo-shaped, slightly deeper than tall, with the triangle clearly visible against the copper.

It is worth comparing the harlequin to the other rasboras it shares shop tanks with, because they are not interchangeable and the price differences are not the only differences. Chili Rasbora (Boraras brigittae, formerly Microrasbora) is much smaller — 1.5–2 cm — bright red with a black side stripe, more delicate, needs softer and more acidic water (pH 4.5–6.5), and schools tightest of the three but in a 10 gallon only with shrimp and tiny fish. The chili is the fish to choose if you want a jewel-box nano with maximum colour per centimetre; the harlequin is the fish to choose if you want a community-tank school that tolerates your tap water. Scissortail Rasbora (Rasbora trilineata) is much larger — 10–12 cm — silver with black-and-yellow forked tail, hardy, schools loose, and needs a 30+ gallon because of the adult size. The scissortail is the fish to choose if you want a rasbora for a larger tank; the harlequin tops out at 5 cm and is the right rasbora for the 10–20 gallon range. The harlequin sits between the two: bigger and hardier than the chili, smaller and tighter-schooling than the scissortail.

Common Health Issues

Harlequin Rasboras are hardy — genuinely hardy, not "hardy for a nano fish" hardy. The main health issues I see are ich (almost always from temperature swings during shipping), flukes (gill flukes show up as rapid breathing and clamped fins, usually from a new fish), and emaciation (when housed with faster-feeding tank mates that outcompete them at mealtime). They are not disease-prone and a properly cycled, properly maintained tank will keep a school healthy for 5–8 years — harlequins are among the longest-lived nano schooling fish, which is one of the underrated things about them.

Ich is treated with the standard heat-and-salt or formalin/malachite protocol. Harlequins are fully scaled and tolerate salt well — 1 tablespoon per 5 gallons for 14 days is the textbook treatment, raising temperature to 28°C (do not exceed 30°C — harlequins stress above that). Formalin-based medications (Rid-Ich, Kordon Rid-Ich Plus) work at the label dose. Copper-based treatments are not toxic to harlequins the way they are to scaleless fish, but I still avoid them in a community tank — if you have shrimp or loaches, copper kills them. Gill flukes respond to praziquantel (Prazipro) at the label dose for 7–10 days; quarantine new fish for 4 weeks in a hospital tank to catch flukes before they enter the display tank.

Prevention is straightforward: weekly 25–30% water changes with dechlorinated, temperature-matched water; no ammonia, no nitrite, nitrate below 20 ppm; stable temperature; and a mature, cycled tank. Harlequins do not belong in a freshly-cycled aquarium — the biofilm and micro-crustacean population of a 3-month-old tank is part of what keeps them fat between feedings. They are schooling fish and a single harlequin will pine and die in isolation — always keep 6 or more. Lifespan in good conditions is 5–8 years, which is longer than most tetras (3–5) and much longer than most Boraras rasboras (2–3). If your harlequins are dying within the first 6 months, look at water quality and tank mates first; the species is robust, so a die-off is almost always an environmental problem rather than a species problem.

Breeding

Harlequin Rasboras are egg-scatterers with a twist: they spawn on the undersides of broad-leaved plants, which is the behaviour that got them reclassified to Trigonostigma. Sexing is straightforward: males are slimmer, more intensely coloured (deeper copper, sharper black triangle with a slightly rounded lower edge), and turn a deeper copper when in spawning condition. Females are fuller-bodied, especially when gravid, with a straighter-edged black triangle. Condition a group with frozen daphnia, frozen brine shrimp, and live baby brine shrimp for two weeks; the females will visibly fill out and the males will start sparring and displaying.

Spawning happens in the morning. The male courts a female by swimming alongside her in a tight arc, and the pair moves to the underside of a broad leaf (Anubias nana, Cryptocoryne wendtii, Echinodorus bleheri — the leaf must be wide and angled). The pair inverts, belly-up, and the female releases 6–12 eggs onto the underside of the leaf while the male fertilises them simultaneously. The eggs are adhesive and stick to the leaf. The pair may repeat the process several times across different leaves over an hour or two; a full spawn is 50–100 eggs across multiple leaves. Remove the adults immediately after spawning — both will eat eggs and fry given the chance. Eggs hatch in 24–36 hours at 26°C; the fry are tiny and free-swimming on day 5–6.

The fry are smaller than tetra fry and need infusoria, green water, or commercial liquid fry food (Sera Micron, SeaChem Paracare Fry Starter) for the first 5–7 days — baby brine shrimp nauplii are too big for the first week. Graduate to vinegar eels at day 7, then BBS at day 10–12. Yields are modest: a single pair spawn might give you 20–30 survivors per spawn if you do everything right, and 0–5 if you get the first food wrong. The trick is to set up multiple spawning tanks and run them in rotation — a conditioned pair will spawn every 7–14 days. Harlequins are harder to breed than the smaller Boraras species but easier than the larger scissortail — they sit in the middle of the rasbora breeding-difficulty curve, which is appropriate given that they sit in the middle of the rasbora size curve too. Fry grow to 1 cm at 8 weeks, sexable at 4 months, sexually mature at 8–10 months.

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