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Trigonostigma heteromorpha, usually sold as the harlequin rasbora, is a peaceful, mid-water schooling fish that thrives in a cycled, planted community aquarium. The most reliable recipe for the vivid copper-pink body and crisp black wedge is a group of 8–10 or more, a tank at least 60 × 30 cm (24 × 12 inches) with a 20-gallon long being more comfortable, stable warm water, gentle flow, dark cover and a varied small-particle diet. “Vibrant harlequin” is not a separate species or recognized morph: color is mainly a result of genetics, health, social security, lighting, substrate and water stability.
What is a harlequin rasbora?
The harlequin rasbora is the deep-bodied Southeast Asian cyprinid Trigonostigma heteromorpha (Duncker, 1904), formerly Rasbora heteromorpha. The genus Trigonostigma was erected by Kottelat and Witte in 1999 for a small group of deep-bodied, wedge-marked “rasboras.” The broad black triangular flank patch and the habit of attaching eggs beneath a broad leaf help distinguish the group.
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Its commonly used names include harlequin, harlequin fish and red rasbora. Shops may also label blue, black, golden (leucistic) or purple forms. These are color varieties of the same species, not different care types; Seriously Fish says the blue, black and golden forms have identical requirements.
In the wild it is associated mainly with southern Thailand, Peninsular Malaysia and Singapore, with records from Sumatra and nearby islands such as Bintan. Northern Sumatra is less certain in the available records, and the Seriously Fish profile does not confirm a single occurrence from Borneo despite older summaries that list it. The fish inhabits gently flowing forest streams and tributaries, including peat-swamp blackwater, where Cryptocoryne, leaf litter, twigs and roots provide shade. Tannins can make the water faintly brown or yellow, with soft, weakly acidic chemistry.
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Almost all aquarium harlequins are commercially farmed. Farmed fish generally tolerate neutral community water better than wild fish, while soft, slightly acidic, stable water and a dark, furnished tank usually produce the deepest color.
How big do harlequin rasboras get?
Seriously Fish lists a maximum standard length of 45 mm (1.8 inches) in 2026. Standard length is measured from the snout to the base of the tail, so it excludes the caudal fin. Aquarium guides often say “about 2 inches” because they are referring to total length; Fishkeeping World reports that most fish reach about 1.75–2 inches and cites a 5 cm FishBase maximum, but that primary FishBase record was not independently checked here. Tropical Fish and Aquariums gives 1.5–1.8 inches (4–4.5 cm), with very few exceeding 2 inches (5 cm), and says full size is usually reached in six to nine months.
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Repair common Windows errors and clear accumulated junk for a smoother, more stable PC - no reinstall needed.Free scan · no reinstallThe short, deep, laterally compressed body makes a harlequin look stockier than a neon tetra of similar length. Females are generally a little larger and deeper-bodied than males.
How long do harlequin rasboras live?
With a mature biological filter, an adequately sized school, good food and stable temperature, hobby sources commonly place aquarium life at roughly five to eight years. Fishkeeping World states, “The lifespan of harlequin rasboras has not been studied systematically, but with good care they commonly live 5 to 8 years in an aquarium.” Tropical Fish and Aquariums centers its estimate at four to six years, with some fish reaching about eight. These are husbandry estimates, not the results of a peer-reviewed lifespan study.
Stressors that shorten life include new-tank ammonia or nitrite, repeated temperature swings, chronic under-grouping, unsuitable tank mates, poor diet and untreated disease. Thin, faded fish that have spent a long time in poor shop conditions may never reach the upper end of the range.
What size tank do harlequin rasboras need?
Use swimming footprint and school size rather than a gallon-per-fish slogan. The smallest base accepted by Seriously Fish is 60 × 30 cm (24 × 12 inches), about 54 litres (approximately 14 US gallons), specifically because this is a schooling fish. A 20-gallon long (about 30 inches/76 cm long) is the more comfortable beginner choice: it gives the school a useful cruising lane and leaves room for calm bottom fish.
| Source | Minimum or preference | Group guidance | What it means in practice |
|---|---|---|---|
| Seriously Fish (revised 2026) | 60 × 30 cm base; about 54 L/14 US gal | At least 8–10 | Smallest defensible footprint when the fish are kept as a proper school |
| Fishkeeping World (2022) | 15 US gal for 8–10; 20 gal preferred; 10 gal too small | 8–10 | At least 24 inches long; 36 inches is more generous |
| Tropical Fish and Aquariums (2026) | 15 gal for eight; 20-gallon long better | 8+, best 10+ | About 10–14 can share a 20-gallon long with suitable bottom fish |
| FishTankMath (updated May 2026) | 20-gallon long, 30 × 12 × 12 inches | Six hard minimum; 8–10 for a full display | Short 10-gallon footprints make a group look cramped and twitchy |
| Glass Box Diaries (indexed 2026) | 75 L/about 20 US gal or larger in its prose | At least eight | Prefer the prose recommendation; the same page’s smaller stocking table conflicts with it |
Tall hexagonal or column tanks waste volume above and below a fish that normally occupies the middle water band. A long rectangular tank is a better use of space. Leave open water across the front and sides, with plants and wood concentrated toward the back and edges.
How many harlequin rasboras should be kept together?
Keep at least 8–10, with 10 or more giving the most natural, relaxed display. Seriously Fish is explicit: “It’s a schooling species by nature and really should be kept in a group of at least 8-10 specimens.” A pair or trio commonly hides, fades and fails to show the loose mid-water school that makes the species appealing.
Some 2026 hobby pages call six a hard minimum. That can keep fish alive in a suitable aquarium, but it is a compromise rather than the strongest welfare or display recommendation. FishTankMath summarizes the distinction as, “The mainstream consensus is six as a hard minimum and eight to ten for the reliable display.” Never use an old “two rasboras per gallon” rule; it ignores footprint, filtration, companions and the need for a real group.
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Can harlequin rasboras live in a 5 gallon tank?
No. A 5-gallon tank cannot provide a proper school and stable swimming space for this species. Tropical Fish and Aquariums explicitly considers 5 gallons too small, and even a 10-gallon tank is described as a short-term or heavily constrained option rather than a good permanent home. Choose a longer tank that can hold at least 8–10 fish and maintain zero ammonia and nitrite.
How many harlequin rasboras can you keep in a 20 gallon tank?
A 20-gallon long can comfortably support a school of 8–10 and, with mature filtration and compatible bottom-dwellers, around 10–14 according to Tropical Fish and Aquariums. Stock the lower end if the tank is heavily planted, has a short footprint, or includes other active fish. Add the school only after the nitrogen cycle is complete, then increase numbers gradually while checking ammonia, nitrite and nitrate.
Harlequin rasbora (Trigonostigma heteromorpha): what water temperature does it need?
The specialist range is 21–28 °C (70–82 °F), pH 5.0–7.5 and hardness 1–12 dGH (Seriously Fish, 2026). For an ordinary community aquarium, the middle is easier to hold: about 22–26 °C (72–79 °F), pH 6.0–7.0 and GH 2–10. Farmed fish can live at pH 5.5–7.5 if it stays steady; color often suffers when alkaline water, hard substrate or abrupt changes push conditions toward the edge.
| Parameter | Species range | Practical community target | Non-negotiable safety point |
|---|---|---|---|
| Temperature | 21–28 °C (70–82 °F) | 22–26 °C (72–79 °F) | Avoid rapid swings; use an adjustable heater in most homes |
| pH | 5.0–7.5 | 6.0–7.0 when achievable without chemicals | Stability matters more than an exact number |
| Hardness | 1–12 dGH | 2–10 dGH | Soft to moderately soft water brings out color |
| Ammonia | 0 ppm | Any detectable reading requires action | |
| Nitrite | 0 ppm | Any detectable reading requires action | |
| Nitrate | Preferably below 20 ppm | Control with maintenance and stocking, not sudden chemical fixes | |
Do not chase pH with bottled “pH up” or “pH down.” Glass Box Diaries puts it plainly: “Don’t use pH-altering chemicals to hit an exact number.” Test your source water, keep the same conditions at every water change, and let the aquarium’s biological and botanical processes remain stable.
For a breeding setup, move toward pH 5.0–6.0, hardness 1–5 °H and the warmer end of the range. Tropical Fish and Aquariums gives a breeding target of 26–28 °C (79–82 °F), pH 5.5–6.5, GH 1–3 and KH 0–2.
How should you set up a harlequin rasbora aquarium?
Build the aquarium around cover, gentle current and an uninterrupted mid-water lane:
- Cycle first. Never add harlequins to an uncycled aquarium. Confirm ammonia and nitrite are 0 ppm before stocking.
- Use a dark, fine substrate. Dark sand or fine dark gravel increases contrast and helps copper-orange color show. Avoid sharp gravel.
- Add Southeast Asian-style cover. Arrange wood roots, branches and dried beech, oak or Ketapang/Indian almond leaves. Leaves can tint the water and support microbial grazing; replace them as they break down.
- Plant the margins. Cryptocoryne, Java fern (Microsorum), Java moss (Taxiphyllum) and Anubias tolerate subdued light. Floating plants such as frogbit or salvinia break glare and make a school feel secure.
- Leave open water. Keep the front and center clear for cruising. Dense plants everywhere can remove the swimming lane the fish need.
- Filter gently. A sponge filter, spray bar or baffled hang-on-back works well. Seriously Fish suggests roughly 4–5 times tank volume per hour; practical guides recommend the low end of turnover and diffusing any jet that pins fish in a corner.
- Control light and temperature. Dim to moderate light is closer to the shaded habitat. FishTankMath suggests capping light at about 6–8 hours while plants establish. Most homes need a heater to prevent winter cooling.
- Fit a lid. Harlequins are not habitual jumpers, but a startled school can leap during transport, sudden light changes or chasing.
Perform a 20–25 percent weekly water change with temperature-matched, dechlorinated water. Fishkeeping World recommends 25 percent weekly and warns that monthly changes are inadequate for a school. A liquid test kit is more useful for diagnosis than strips; Glass Box Diaries names the API Master Test Kit for ammonia, nitrite, nitrate and pH and API Tap Water Conditioner as one dechlorinator example. Other liquid kits and conditioners work if they remove chlorine and chloramine.
What do harlequin rasboras eat?
They are micropredators with small mouths. Stomach analyses of wild fish found small insects, worms, crustaceans and other zooplankton; Seriously Fish states, “Stomach analyses of wild specimens have revealed it to be a micropredator feeding on small insects, worms, crustaceans and other zooplankton.”
Use a quality micro-pellet or crushed flake as the staple. Named examples in the care guides include Hikari Micro Pellets, Fluval Bug Bites or Bug Bites Microgranules and Tetra Color flakes. Alternate brands if you wish; particle size matters more than the label. Add frozen or live foods such as bloodworm, Daphnia, Artemia/baby brine shrimp, cyclops and finely chopped mysis two or three times a week for condition and color.
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- Feed once or twice daily, offering only what the school can finish quickly. Sources use windows from about 30 seconds to two or three minutes; the shared rule is no food left on the bottom.
- Use daphnia or baby brine shrimp for small fish and fry; very large bloodworms are a poor fit for tiny mouths.
- Thaw frozen food before feeding and remove uneaten pieces.
- One 2026 guide recommends a fasting day each week for adult fish; it is optional if portions are already controlled.
- For breeding, condition adults with small live and frozen meals two or three times daily for roughly one to four weeks, depending on the guide.
Flake-only fish can survive, but the guides consistently associate mixed micro-foods and frozen foods with stronger color and breeding condition. Botanicals provide grazing and microbial life, not a substitute for feeding.
Are harlequin rasboras aggressive?
No. They are peaceful community fish. Seriously Fish says, “This species is very peaceful indeed making it an ideal resident of the well-maintained community tank and an unreserved recommendation for those new to fishkeeping.” Males may make brief display chases, but these contests normally cause no injury and can intensify color.
Choose tank mates that are similarly small, calm and tolerant of warm, soft-to-neutral water: neon, cardinal, ember or chili tetras; pygmy corydoras and other peaceful corydoras; otocinclus; kuhli loaches; honey gouramis; small peaceful barbs; and appropriate snails. Adult amano or cherry shrimp are generally safer than shrimplets, which may be eaten.
Avoid adult angelfish, aggressive or large cichlids, tiger barbs, serpae tetras and anything large enough to swallow a 4–5 cm fish. Very slow or timid species may be stressed by a busy school. Guppies, mollies and platies can share a neutral middle ground, but their preferred harder, more alkaline water may leave harlequin color less intense.
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Do harlequin rasboras and neon tetras school together?
No. They may swim in loose proximity and copy one another’s direction, but they do not form one true mixed school. FishTankMath states, “No. They cohabit peacefully and pick up some loose coordination, but they don’t form a true mixed school — each species needs its own group of 6+ to express normal behavior.” Keep a full harlequin group and a separate neon group rather than counting one species toward the other’s minimum.
Can harlequin rasboras live with bettas?
Sometimes, but never as a guaranteed pairing. A calm betta can work in a planted aquarium of about 20 gallons or more because harlequins stay in mid-water and move quickly, yet individual bettas vary. Provide dense plant cover, break up sight lines, add the school before the betta where possible, and observe closely during the first days. Remove either fish if there is persistent chasing, fin damage or hiding.
How do you tell a male harlequin rasbora from a female?
The most reliable clue is body shape: mature females are usually rounder-bellied and often a little larger, while males are slimmer and more colorful. Seriously Fish states, “Mature females are usually rounder-bellied and often a little larger than the slimmer, more colourful males.”
The black wedge is a secondary clue and is described slightly differently by sources. Seriously Fish and Tropical Fish and Aquariums describe a male’s lower front corner as sharper or more pointed; females tend to have a straighter, rounder-looking edge. Aquarist Atlas also describes males as slimmer with a more extensive wedge and females as deeper-bodied, especially when carrying eggs. Young fish under about six to nine months can be difficult to sex, so buying a group of 10 is more dependable than trying to select a pair.
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They can breed at home, but they are more demanding than danios or livebearers because the adults eat the eggs and the fry need very small first foods. In a mature planted community tank, occasional fry may appear, but most are eaten.
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For a deliberate spawn:
- Prepare a dim breeding tank. Use a shallow, separate aquarium with very soft acidic water, a small air-powered sponge filter and a mesh, plastic grass mat or bare base that lets eggs fall out of reach. Add Cryptocoryne, Microsorum, another broad leaf or an artificial spawning surface.
- Set the chemistry. Aim for pH 5.0–6.0, hardness 1–5 °H and temperature toward 26–28 °C. Keep nitrate very low.
- Condition adults. Feed small live and frozen foods two or three times daily for one to four weeks. Select mature, well-fed fish; older fish are often more productive.
- Introduce breeders. When females are full and males displaying, use one pair or about two males per female. A large 40–50 percent temperature-matched water change in the evening can trigger morning spawning.
- Watch the leaf. The pair often turns upside down beneath a broad leaf and attaches eggs to its underside. Do not feed adults in the spawning tank.
- Remove adults or eggs immediately. “Post-spawning the adults will eat any eggs they find so either they or the eggs themselves should be removed as soon as possible,” says Seriously Fish.
- Raise fry correctly. Seriously Fish gives incubation of usually 24–48 hours and free swimming in about a week. Other hobby guides report 18–36 hours and free swimming in three to five days, so timing varies with temperature and development. Start with Paramecium or other infusoria, then add microworms and newly hatched Artemia when fry can take them.
Published clutch estimates should not be treated as a species constant: reports range from a few dozen eggs, to up to 80–100, while one 2026 guide gives 80–300. The consistent point is that eggs arrive in small batches beneath leaves and that soft, clean, acidic water matters more than specialized equipment.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Why are my harlequins pale?
Pale color is usually a husbandry or stress signal before it is a disease diagnosis. Check in this order:
- Water quality. Test ammonia, nitrite, nitrate, pH and temperature. Correct any ammonia or nitrite immediately with appropriate water changes and verify the cycle.
- Group size. A pair, trio or undersized school often hides and loses color. Move toward 8–10 or more, provided the tank footprint supports them.
- Lighting and substrate. Bright light and pale gravel reduce visual contrast. Dark sand, floating shade and a planted background commonly make the same fish look much richer.
- Parameter drift. Hard, alkaline water, crushed coral, neglected tap-water changes or sudden shifts can wash out the black wedge and orange body. Stabilize rather than dosing pH chemicals.
- Stress or illness. Check for clamped fins, hiding, loss of appetite, white spots, gold dust, bloating, fin erosion or surface hanging. Quarantine new arrivals for two to four weeks.
Color may remain muted for a week or two after transport or a parameter change. Improve stability, flow, cover and diet before reaching for medication. Use only a product labeled for the diagnosed condition and the actual aquarium volume; disease-specific antibiotic or ich dosing should come from the product directions and an aquatic veterinarian or qualified local professional.
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Common problems listed by the care guides include ich (white spots), velvet (fine gold dust), fin rot, columnaris, dropsy and internal parasites. New-tank syndrome, cold swings and aggressive tank mates are frequent underlying causes.
- Ich: isolate if possible, test and stabilize water, increase aeration and use a medication labeled for small scaled fish. Some hobby guides suggest heat, but heat alone is not a universal treatment protocol.
- Fin rot: correct water quality first; use an antibacterial treatment only if damage continues.
- Dropsy: pinecone-like swelling has a poor prognosis. Quarantine and seek professional advice promptly.
- Columnaris or unexplained wasting: act quickly, quarantine and obtain an accurate diagnosis rather than guessing at doses.
Observe the school daily. A healthy group is active in mid-water, accepts food and shows a copper-pink body with a clean black patch when settled.
How are harlequin rasboras different from lambchop and glowlight rasboras?
Shops often mix the names, but the fish are different species with broadly similar care. The true harlequin, T. heteromorpha, is the deepest-bodied of the common wedge rasboras and has the broadest, darkest triangular mark. Lambchop rasboras (T. espei) are slimmer with a narrower patch that resembles a lamb chop; glowlight rasboras (T. hengeli) are smaller and show a thin glowing stripe above the patch.
Compare body depth, the width and darkness of the wedge, adult size and the shop’s scientific label rather than assuming every orange wedge rasbora is a harlequin. Care overlaps, but the name and adult size do not.
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Blue, black and golden/leucistic fish are sold as forms of the same species, and Tropical Fish and Aquariums also treats purple fish as T. heteromorpha. They can school together and use the same temperature, water, food and tank-size plan. “Vibrant” is a marketing description, not a documented separate morph.
Color is affected by genetics, mood, age, social security, diet, substrate and light. Wild fish can be more intensely colored than mass-farmed fish; a large, settled group in a dim, dark-bottomed aquarium often looks more vivid than isolated shop fish.
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Is the harlequin rasbora endangered?
Secondary sources report the harlequin rasbora as Least Concern (LC) on the IUCN Red List; no assessment year or criteria code was available in the supplied records. Aquarist Atlas states, “Trigonostigma heteromorpha is listed as Least Concern (LC) on the IUCN Red List.” That status does not mean every local population is secure.
Peat-swamp forests and blackwater streams in the Malay Peninsula and Singapore have been drained for agriculture, plantations and development. The aquarium trade is now supplied mainly by commercial captive breeding, reducing collection pressure, but habitat loss can still threaten local wild populations. Buy healthy fish, avoid releasing aquarium animals and maintain them for their full potential lifespan.
What is the simplest reliable care plan?
Choose a cycled 20-gallon-long aquarium, add 8–10 harlequins, keep 22–26 °C water with zero ammonia and nitrite, use a dark planted layout with floating shade, diffuse the filter outlet, feed a small-particle staple plus frozen foods and change 20–25 percent of the water weekly. If the fish are pale, test water and review group size, light, substrate and tank mates before changing chemistry. For breeding, move to a separate dim tank with very soft acidic water and remove adults as soon as eggs appear.
Frequently Asked Questions
What does “standard length” mean for a harlequin rasbora?
Standard length is measured from the tip of the snout to the base of the tail, excluding the caudal fin. That is why a 45 mm standard-length fish can be described in hobby guides as roughly 2 inches overall.
Can harlequin rasboras live without an aquarium heater?
Only if the room keeps the aquarium reliably within their 21–28 °C range through the coldest night. Glass Box Diaries notes that most homes need a heater to prevent winter cooling, so verify temperature with a thermometer rather than relying on room temperature.
Should I quarantine harlequin rasboras before adding them to my display tank?
Yes. Tropical Fish and Aquariums recommends quarantining new fish for two to four weeks. This gives you time to observe appetite, breathing, spots and wasting before a problem reaches the established school.
Can harlequin rasboras eat only flakes?
They can maintain themselves on a correctly sized quality flake, but the care guides associate stronger color and breeding condition with micro-pellets or crushed flakes supplemented by foods such as daphnia, baby brine shrimp or bloodworm. Particle size and portion control are more important than a particular brand.
What does Least Concern mean for this fish?
Least Concern is an IUCN risk category indicating that the species is not currently placed in a threatened category at the global assessment level. It does not guarantee that every stream or peat swamp supporting local populations remains intact.
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