--- type: species title: "Harlequin Rasbora Care Guide: Tank Setup, Diet & Tank Mates" slug: "harlequin-rasbora" category: "freshwater" scientificName: "Trigonostigma heteromorpha" subcategory: "Rasbora" lastUpdated: "2026-04-24" readingTime: 10 url: https://www.fishstores.org/species/harlequin-rasbora --- # Harlequin Rasbora Care Guide: Tank Setup, Diet & Tank Mates *Trigonostigma heteromorpha* Learn how to care for harlequin rasboras — water parameters, tank mates, feeding, and breeding tips for this iconic community fish. ## Species Overview Harlequin rasboras (*Trigonostigma heteromorpha*) are a small, copper-orange schooling fish from the soft, tannin-stained streams of the Malay Peninsula, Thailand, and Sumatra. They've been a community-tank staple since the early 1900s, and their popularity has never really faded — they're hardy, beginner-friendly, and the wedge-shaped black patch along the back half of the body makes them instantly recognizable in any planted aquarium. Few schooling fish offer this much visual payoff in a 10-gallon footprint. Most aquarists keep them for the shoaling behavior. A tight school of 8 or 10 fish drifting through midwater against a backdrop of dark substrate and floating plants is one of the classic looks of the planted-tank hobby. They're also forgiving of typical tap-water conditions in most US cities, which makes them a sensible first schooling fish for hobbyists moving past the betta stage. | Field | Value | | ----------- | -------------------------- | | Adult size | 1.75 in (4.5 cm) | | Lifespan | 5-8 years | | Min tank | 10 gallons (school of 6-8) | | Temperament | Peaceful | | Difficulty | Beginner | | Diet | Omnivore | ### Natural Habitat In the wild, harlequin rasboras inhabit slow-moving forest streams and peat swamps across the Malay Peninsula, southern Thailand, and Sumatra. The water is heavily shaded by rainforest canopy, stained tea-brown by tannins from decaying leaf litter, and consistently soft and acidic. Typical wild parameters sit around pH 5.5-6.5 with very low mineral content. These habitats are dense with submerged roots, fallen wood, and broad-leafed plants like *Cryptocoryne*, which the species depends on for spawning. Replicating even a stripped-down version of this habitat — dark substrate, leaf litter, gentle flow — produces noticeably better color and behavior than a bare community setup. Wild populations have declined in some parts of their range due to habitat loss, but the trade is dominated by tank-bred fish from commercial farms in Southeast Asia and Eastern Europe. Almost every harlequin rasbora sold in the US is captive-raised. ### Appearance & the Distinctive Black Triangle Patch The body is a warm copper-orange that shifts toward pinkish-rose in well-fed adults, and the defining feature is the solid black patch on the rear half of the body. Hobbyists call it the "pork chop" or "lambchop" patch because of its wedge shape — wider at the top, tapering toward the caudal peduncle. It's the single most reliable identifier for the species. > **Spotting a true Harlequin in a mixed tank** > > The black patch sits on the rear half of the body and looks like a cleaver or pork chop. If the patch is narrow, broken, or runs the full length of the body, you're probably looking at the lambchop rasbora (*T. espei*) or hengeli rasbora (*T. hengeli*) — close relatives commonly mixed into shipments. Sexual dimorphism is subtle but learnable. Males have a slightly more angular, pointed front edge to the black patch, often with a small extension along the bottom margin. Females are a touch deeper-bodied and the patch base is more rounded. Both sexes show their best color against dark substrate and dim, warm-toned lighting. ### Size & Lifespan Adults reach about 1.75 inches (4.5 cm) total length, with most store-bought fish topping out around 1.5 inches. They're a true nano-friendly species — a school of 8 takes up less biological footprint than a single fancy goldfish. Growth from a 1/2-inch juvenile to full adult takes roughly 6 months under good conditions. Lifespan in captivity averages 5-8 years, with the upper end achievable when water parameters stay stable and the school is large enough to keep stress low. Most premature deaths come from being kept in groups of 3-4 (chronic stress), exposure to nitrates above 40 ppm for extended periods, or temperature spikes during summer in unheated rooms. ## Water Parameters & Tank Requirements Harlequins are flexible by rasbora standards — they tolerate moderately hard, neutral water that would crash more sensitive blackwater species. That said, they show their best color, behavior, and breeding readiness in soft, acidic, tannin-stained conditions that match their wild habitat. ### Ideal Water Parameters Target temperature is 72-82°F (22-28°C), with most keepers settling around 76-78°F for everyday display tanks. They handle short excursions outside this range, but sustained temperatures above 82°F shorten lifespan and suppress appetite. For pH, aim for 6.0-7.5. They breed best below 6.5, but they'll live full lives at pH 7.0-7.4, which is what most US municipal water sits at after off-gassing. General hardness should fall between 2-10 dGH; soft water (under 5 dGH) brings out the most intense coloration and is required for serious breeding work. Ammonia and nitrite must read zero. Keep nitrate under 20 ppm with weekly 25-30% water changes. They're more sensitive to nitrate than tetras of similar size — chronic exposure to 40+ ppm is one of the leading causes of fading color and shortened lifespan in this species. ### Minimum Tank Size & Schooling Space A 10-gallon tank is the practical floor for a school of 6-8 harlequin rasboras with no other fish. This works because they're small and stay in midwater, but it leaves no room for additional species or for the school to truly stretch out and display natural shoaling behavior. For most setups, a 20-gallon long is the better choice. The 30-inch footprint gives a school of 10-12 enough lateral swimming space to actually behave like wild fish — tight formations, coordinated direction changes, and the chasing-and-darting that males do when displaying. A [20-gallon fish tank](/guides/20-gallon-fish-tank) also opens the door to compatible community species like corydoras or small tetras. If you have the option, choose length over height. These fish swim horizontally and almost never use vertical space above the midwater zone. ### Filtration, Flow & Décor Filtration should be gentle. A sponge filter rated for 15-20 gallons or a small hang-on-back with the output baffled by a sponge or spray bar handles a school of 8-12 fish without blasting them around the tank. Wild habitat is slow-moving forest stream — strong powerheads stress them and disrupt natural shoaling. Décor matters more than equipment. Dark substrate (black sand or dark gravel) makes their orange body color pop in a way that pale gravel never will. Add Indian almond leaves or alder cones to the tank to release tannins; the resulting amber tint is closer to wild conditions and contains compounds that mildly inhibit fungal and bacterial growth on fins and eggs. > **Floating plants are non-negotiable** > > Harlequin rasboras come from canopied forest streams — direct overhead lighting stresses them. Add a floating plant layer (frogbit, dwarf water lettuce, salvinia) to break up the light. Within a week of adding floaters, you'll see the school spend more time out in the open and show deeper color. Live plants also serve a practical purpose: broad-leafed species like *Cryptocoryne wendtii* and *Anubias* are the natural spawning substrate for this species. Even if you don't plan to breed them, a few crypts give the school cover and territorial reference points. ## Diet & Feeding Harlequin rasboras are opportunistic micropredators — they evolved to pick small invertebrates and zooplankton out of the water column and off submerged surfaces. In captivity they'll eat almost anything that fits in their small mouths, which makes feeding straightforward. ### What Harlequin Rasboras Eat in the Wild Stomach-content analysis of wild specimens shows a diet of micro-crustaceans (cyclops, daphnia), insect larvae (mosquito and midge larvae), small worms, and zooplankton. They're surface-and-midwater feeders, rarely picking food off the substrate. This wild diet is high in protein and fat with some plant matter and bacterial film mixed in. Recreating that nutritional profile in captivity comes down to rotating between high-quality dry food and frequent frozen or live offerings. ### Recommended Foods & Feeding Schedule Build the daily diet around a quality micro pellet or crushed flake — Hikari Micro Pellets, Bug Bites Micro, and TetraMin Tropical Flake are all proven options. Pellets should be small enough to fit into a mouth roughly 1-2mm wide; anything larger gets spit out. Supplement 2-3 times per week with frozen daphnia, baby brine shrimp, or finely chopped bloodworms. Live foods (where available) trigger the strongest color and breeding response — daphnia in particular is closer to their natural diet than any prepared food. Vinegar eels and microworms are good live-food cultures to maintain if you're considering breeding. Feed small amounts 1-2 times daily, no more than the school can finish in 90 seconds. The "2-minute rule" is generous for a species this small — they have tiny stomachs and uneaten food fouls water fast in the small tanks they're typically kept in. ## Tank Mates & Compatibility Harlequin rasboras are model community fish: peaceful, mid-water dwelling, fast enough to escape mild aggression, and small enough that even semi-aggressive species rarely target them. They're compatible with almost any other small, peaceful tropical fish. ### Best Community Tank Mates Top compatible species: - **Corydoras catfish** (panda, pygmy, sterbai) — bottom dwellers that occupy completely different territory - **Ember tetras, neon tetras, cardinal tetras** — similar size, similar temperament, similar water preferences - **Chili rasboras** (*Boraras brigittae*) — sister rasbora that schools beautifully alongside harlequins; see the [chili rasbora care guide](/species/chili-rasbora) for matched parameters - **Galaxy rasboras** (celestial pearl danios) — adds visual contrast with similar peaceful temperament; see the [galaxy rasbora care guide](/species/galaxy-rasbora) - **Otocinclus catfish** — algae-eating bottom dwellers, completely peaceful - **Dwarf gouramis, sparkling gouramis, honey gouramis** — slow-moving surface dwellers that don't compete with rasboras in midwater For a more classic comparison, the [neon tetra](/species/neon-tetra) is the closest South American analog to the harlequin rasbora — same size class, same schooling instinct, same beginner difficulty. The two species often share tanks and ignore each other. ### Species to Avoid Avoid fin-nipping species — tiger barbs, serpae tetras, and Buenos Aires tetras will harass a slow-moving school and shred fins. Skip large or aggressive cichlids (oscars, jack dempseys, convicts), which view a 1.75-inch rasbora as a snack. Goldfish are a hard no on multiple grounds: temperature mismatch, size disparity, and the rasboras will get sucked into the goldfish's large mouth. Bettas are a case-by-case call. Most male bettas tolerate harlequin rasboras because the rasboras are too fast to catch, don't have flowing fins to provoke aggression, and stay in midwater while the betta patrols the surface. But individual bettas vary, and an aggressive male will exhaust the school by chasing them constantly. Have a backup plan. ### Keeping Them in Schools The minimum school size is 6, the recommended size is 8-10, and 12+ produces the most natural behavior. Below 6 fish, harlequins show measurable stress: clamped fins, reduced feeding, hiding behavior, and faded color. The orange goes from copper-rose to pale peach within a week of being moved to a too-small group. > **Don't undersize the school** > > A school of 3-4 harlequin rasboras is not a school — it's a stressed group of solitary fish. They'll survive but won't thrive, and the visual payoff (the entire reason most people buy them) is nearly absent. Always buy at least 8 at once, ideally 10. Smaller schools can be merged into a single larger one with no aggression issues. In a properly sized school, you'll see fish swim in tight formation, change direction together, and engage in the subtle male-male dominance displays that produce the most intense coloration. This is what you're paying for — get the numbers right. ## Breeding Harlequin Rasboras Harlequins breed readily in the home aquarium, but they require specific conditions to spawn and the fry are notoriously difficult to raise. They're also unique among common community fish for spawning on the underside of broad leaves rather than scattering eggs over the substrate. ### Conditioning & Spawning Triggers Pick out the largest, most rounded females and the most colorful males from your existing school — usually 3-4 of each. Move them to a 10-gallon breeding tank with very soft, acidic water (pH 6.0-6.5, under 4 dGH) and bump the temperature to 80-82°F. Drop the water level to 6-8 inches deep. > **Broad-leaf plants are essential for spawning** > > Unlike most egg-scattering rasboras, harlequins lay eggs on the underside of broad leaves. Stock the breeding tank with *Cryptocoryne wendtii*, *Cryptocoryne walkeri*, or large-leafed *Anubias* — the female will swim upside down beneath the leaf during spawning. No leaves, no spawning. Plastic plants don't work; the female chooses based on leaf texture. Condition the breeders with live or frozen foods (daphnia, baby brine shrimp) for 7-10 days before introducing them to the breeding tank. A small water change with cooler, soft water often triggers spawning the next morning. ### Egg-Laying Behavior on Broad Leaves When ready, pairs swim together along the underside of a chosen leaf in a coordinated S-curve display. The female deposits 6-12 eggs per pass, the male fertilizes them immediately, and the cycle repeats. A single female can lay 80-120 eggs across a few leaves over the course of an hour. Remove the adults immediately after spawning — they will eat the eggs and any fry that hatch. Eggs hatch in 18-24 hours, and fry become free-swimming after another 3-5 days. Feed infusoria or commercial liquid fry food (Hikari First Bites) for the first week, then switch to baby brine shrimp once they're large enough to handle them. Survival rates are typically 30-60% under attentive care. ## Common Health Issues Harlequin rasboras are hardy by tropical-fish standards, but they're vulnerable to the same parasitic and bacterial issues that affect most small community fish. The two diseases to watch for are ich and fin rot. ### Ich & Velvet Ich (*Ichthyophthirius multifiliis*) presents as small white salt-grain spots on the body and fins, often paired with flashing (rubbing against decor) and labored breathing. It usually appears after temperature swings or stress from new tank introductions. The standard treatment — raising the temperature to 82-84°F for 7-10 days — works on harlequin rasboras and is preferred over heavy chemical dosing. They're sensitive to copper and formalin at low pH, which is exactly the water chemistry they prefer. If you must medicate, use ich treatments at half the recommended dose for the first 24 hours and watch for adverse reactions. Velvet (*Oodinium*) is rarer but more dangerous, presenting as a fine gold-dust film on the body. Treatment requires darkening the tank (the parasite is photosynthetic) plus copper-based medication in a separate hospital tank. ### Fin Rot & Bacterial Infections Fin rot shows as fraying, white edges, or visible erosion on the tail and dorsal fins. It's almost always linked to poor water quality — chronic ammonia or nitrite spikes, or nitrate above 40 ppm sustained for weeks. Catching it early lets you treat with daily 30% water changes and aquarium salt at 1 tsp per 5 gallons; advanced cases need an antibiotic like Furan-2 or Maracyn. Other bacterial infections (mouth rot, body ulcers) follow the same pattern: water-quality breakdown plus stress. The single best preventive measure is consistent weekly water changes and not overstocking the tank. ## Where to Buy & What to Look For Harlequin rasboras are common at almost every freshwater fish store, which is both a blessing and a curse — abundance means easy sourcing, but it also means a lot of stressed, recently-shipped fish hit display tanks and don't recover. ### Selecting Healthy Fish at Your Local Fish Store Look for active schooling behavior in the display tank — healthy fish move together, hold position in the water column, and respond to your presence by tightening the school. Avoid any tank where fish are scattered on the bottom, gasping at the surface, or showing white spots. > **Buy from a tank that's been stocked at least 5 days** > > Newly arrived rasboras often look fine for 24-48 hours and then crash from accumulated shipping stress. Ask the staff when the fish came in — anything less than 5 days old is a gamble. The best stores quarantine new arrivals for a week before putting them on the sales floor. Five visual checks before you buy: 1. **Full color** — copper-orange body with crisp, solid black patch (not faded gray or peach) 2. **No clamped fins** — dorsal and tail should be open and active 3. **Intact fins** — no fraying, white edges, or chunks missing 4. **Clear eyes** — bright, no cloudiness or swelling 5. **Active schooling** — fish should be moving together in midwater, not isolated or hanging at corners Always quarantine new fish for 2-3 weeks before adding them to an established display tank. A 5-gallon tank with a sponge filter and a heater is enough. ### Wild-Caught vs. Tank-Bred Almost all harlequin rasboras sold in the US are tank-bred from commercial farms in Indonesia, Thailand, and the Czech Republic. Wild-caught specimens occasionally show up at specialty stores and online vendors at premium prices. Tank-bred fish acclimate to typical aquarium parameters easily and tolerate moderate water hardness. Wild-caught fish typically show more intense coloration but require softer, more acidic water and longer acclimation periods. For 95% of hobbyists, tank-bred is the better choice — they're cheaper, hardier, and visually nearly identical after a few weeks in a well-maintained planted tank. For broader context on stocking a peaceful planted setup, see our [freshwater fish guide](/guides/freshwater-fish) and the [20-gallon fish tank setup walkthrough](/guides/20-gallon-fish-tank). ## Quick Reference - **Adult size:** 1.75 in (4.5 cm) - **Lifespan:** 5-8 years - **Tank size:** 10 gallons minimum for school of 6-8; 20-gallon long preferred - **Temperature:** 72-82°F (22-28°C); 76-78°F ideal - **pH:** 6.0-7.5; 6.0-6.5 for breeding - **Hardness:** 2-10 dGH; soft water for best color - **Diet:** Omnivore — micro pellets, frozen daphnia, baby brine shrimp, bloodworms - **School size:** 6 minimum, 8-10 recommended, 12+ ideal - **Tankmates:** Corydoras, ember tetras, neon tetras, chili rasboras, galaxy rasboras, otocinclus, peaceful gouramis - **Avoid:** Tiger barbs, large cichlids, goldfish, aggressive bettas - **Difficulty:** Beginner - **Breeding:** Achievable in soft acidic water at 80-82°F with broad-leaf plants like *Cryptocoryne* ## Frequently Asked Questions ### How many harlequin rasboras should be kept together? Keep a minimum school of 6, though 10 or more produces the most natural shoaling behavior. Smaller groups show stress, hiding, and faded coloration. A 20-gallon long tank comfortably houses a school of 10-12 alongside other peaceful community fish. ### Are harlequin rasboras good for beginners? Yes — they tolerate a moderate pH range (6.0-7.5), accept most quality prepared foods, and rarely show aggression. Their main requirement is being kept in schools of 6+. Hardy and forgiving of minor water fluctuations, they're one of the best starter schooling fish available. ### What do harlequin rasboras eat? They accept high-quality micro pellets, flake food, and relish frozen or live foods like daphnia, baby brine shrimp, and bloodworms. Feed small amounts 1-2 times daily. Varied diet enhances color and supports immune health. ### Can harlequin rasboras live with bettas? Often yes — harlequin rasboras are fast, non-nippy, and occupy mid-water, reducing betta conflict. However, monitor the betta's temperament closely. Avoid the pairing with highly aggressive individual bettas, and ensure the tank is at least 10 gallons with plenty of plant cover. ### How long do harlequin rasboras live? With stable water parameters and a varied diet, harlequin rasboras typically live 5-8 years in captivity. Consistent temperature (72-82°F), regular water changes, and low stress from proper schooling numbers are the biggest factors in reaching the upper end of that range. --- *Source: [FishStores.org](https://www.fishstores.org/species/harlequin-rasbora)* *Last updated: April 24, 2026*