Harlequin Rasbora Care Guide

The harlequin rasbora is the textbook peaceful community schooler — a small, pink-bodied fish wearing a bold black wedge, soft-water by birth, and one of the easiest, hardiest centrepiece shoals a planted tank can carry. It has been a hobby icon since the 1930s and remains the fish most often paired with corydoras and tetras for a calm, colourful community.

Harlequin Rasbora at a glance

The sourced figures the welfare engine uses to judge Harlequin Rasbora — the parseable key facts.

Where it comes from

Trigonostigma heteromorpha comes from south-east Asia — peninsular Thailand and Malaysia, Singapore, Sumatra and Borneo, with its type locality in Kuala Lumpur. Its home is shaded, gently-flowing forest streams and peat swamps where Cryptocoryne plants grow thickly, the water soft, weakly acidic and tannin-stained by dissolved humic acids. That blackwater origin explains almost everything about its care: it shows its best colour and least stress in a planted, dimly-lit tank with dark substrate, floating plants, driftwood and botanicals, while a bright, bare tank washes it out. The slow forest-stream heritage means gentle filtration rather than strong current, and the natural association with broad Cryptocoryne leaves is not incidental — those leaves are the spawning substrate.

Did you know?

• It is a leaf-spawner, not a scatterer — almost uniquely among the 'rasboras' it glues its eggs to the underside of a broad leaf, the pair spawning upside-down beneath it.
• The name is literally a costume: 'harlequin' refers to the black triangular patch resembling the diamond pattern on a harlequin jester's outfit.
• It has been a hobby icon since the 1930s, appearing in gold on the cover of William T. Innes's Exotic Aquarium Fishes (1935) across all nineteen editions.
• It gained a sibling in 2020 — Trigonostigma truncata was split off the harlequin complex after turning up in wild-caught aquarium stock, so some shop 'harlequins' are a fish only recently named by science.
• The genus name Trigonostigma means roughly 'triangle mark', and the wedge even has a formal technical name, the 'axine', which in true heteromorpha extends back into the caudal peduncle.
• It is assessed by the IUCN as Least Concern and is one of the most heavily traded community fish, both wild-caught and farmed.

Tank size — and why

A 60 cm footprint, roughly a 15-gallon long, is the right target, with a 10-gallon the tight floor for six only and 20 gallons better for a full school of eight to ten. The driver is not bioload — harlequins are tiny and low-waste — but swimming room and group size. A longer footprint gives a tighter, more confident shoal and steadier water; height matters far less than an open mid-water swimming lane. There is no territorial or oxygen-demand reason for a big tank here, only the welfare of the school.

As a guide, a 20-gallon tank comfortably suits about 8–11 Harlequin Rasbora as a single-species display, leaving room for tankmates.

How big does it really get?

Full-grown Harlequin Rasbora reach about 4.5 cm (1.8 in) long, but they are usually sold at only about 2.5 cm (1 in) — a typical shop size (estimate). At full size, Harlequin Rasbora needs roughly a 15-gallon tank, about 51 cm long; a common 10-gallon starter kit is only about 51 cm.

Adult size is sourced; the shop size is a typical-juvenile estimate; tank length is approximate for a standard 15-gallon aquarium.

Water parameters in practice

Soft, slightly acidic water suits it best, around pH 6–7 and low hardness, though it tolerates neutral and moderately hard tap water perfectly well for everyday keeping — it simply shows better colour in soft water and will only breed in it. Temperature is worth a careful note, because the sources spread: aim for about 23–26 °C. Seriously Fish lists 21–28 °C, FishBase a tighter 22–25 °C, and Wikipedia around 22–27 °C, so treat 28 °C as a tolerated ceiling (and the breeding temperature) rather than the everyday ideal — living permanently at the top of the range is not what you want. Like most soft-water blackwater fish, it is hardy once settled into stable water but more sensitive to unstable parameters and an uncycled tank than to a wide but steady range. Stability beats chasing perfect numbers.

Diet & feeding

An unfussy omnivore. In the wild it is a micropredator taking worms, small crustaceans, insects and zooplankton; in the tank it enthusiastically accepts prepared foods, so a good-quality micro flake or small pellet makes the staple. Vary it with small live and frozen foods — bloodworm, daphnia, brine shrimp, mosquito larvae — to deepen colour and to condition for breeding. Feed small amounts once or twice a day; the mouths and stomachs are tiny, so sparing feeding protects water quality. As an active mid-water feeder it can be out-paced by faster or surface fish, so make sure it gets its share.

Gear & setup

A planted, dimly-lit setup over dark substrate is ideal — densely planted with broad-leaved Cryptocoryne and Aponogeton (also the natural spawning leaves), Amazon sword, floating plants for shade, plus driftwood and leaf litter to tint the water. A gentle filter and modest flow match the slow forest stream it comes from; avoid strong current. No heater question here in the sense of needing one — it is a tropical fish that does want a heater to hold a stable mid-20s temperature. A lid is sensible as with any active small fish, but harlequins are not noted jumpers.

Temperament & behaviour

Peaceful to a fault. This is a non-territorial, non-nipping mid-water shoaler that asks nothing of its tankmates and spends its day schooling in open water. There is essentially no aggression — not toward other species and not within its own group. The behaviour only appears properly in numbers: a tight, confident, colourful shoal in a good-sized group, versus skittish, washed-out, hiding fish when kept in ones and twos. It is the definitive 'centrepiece shoal' for a calm planted community.

Group & social needs

Keep a real group. Five is the floor cited by FishBase and six the common minimum, but the species looks and behaves far better at eight to ten or more, which is what to aim for. Bigger groups cut stress and timidity, tighten the schooling, improve colour, and dilute any one fish being singled out. Under-stocked harlequins hide and fade; treat six as the bare minimum and buy eight to ten.

Compatible tank mates (preview)

A short, engine-cleared shortlist — the species TankStocking's welfare engine clears with Harlequin Rasbora and that suit its size and temperament best. Tap any to load the pairing in the planner.

• Amano Shrimp — Peaceful temperament, similar adult size.
• Black Neon Tetra — Peaceful temperament, similar adult size.
• Black Phantom Tetra — Peaceful temperament, similar adult size.

A note on the shrimp and snails here: Harlequin Rasbora is peaceful and generally invertebrate-safe — but almost any fish will take very small shrimplets given the chance, so give shrimp dense cover (moss, leaf litter) if you want a colony to grow, rather than expecting every baby to survive.

See the full Harlequin Rasbora tank mates guide →

Breeding & sexing

This is the species' signature trick and a genuine rarity among 'rasboras' — it is a leaf-spawner, not an egg-scatterer. The pair spawn upside-down beneath a broad plant leaf: the female swims inverted under a Cryptocoryne or Aponogeton leaf and rubs her belly along it while the male, also inverted, curls his tail around her with a trembling motion to fertilise the adhesive eggs as she lays them. Sexing is subtle — females are rounder-bellied and a little larger, males slimmer and more colourful, and a finer cue is the black wedge, which has a sharper outline in males and a rounder one in females. Breeding is rated moderate-to-advanced and is often called notoriously difficult at home, almost entirely because of strict water chemistry: a separate, dim, soft, acidic tank around pH 6.0–6.4, hardness no higher than about 4 dGH and roughly 28 °C, with the pair heavily conditioned on live foods first. They will not spawn in hard, alkaline water, and egg fertility suffers even if they do. Expect a handful of eggs at a time building toward 80–300 over a spawning; eggs hatch in roughly 18–36 hours. Remove the adults, as they will eat eggs and fry, and start fry on infusoria then newly-hatched brine shrimp.

Lifespan

Long-lived for a small community fish — commonly five to eight years, with many reaching about six, which is notably longer than the two-to-five years of a neon tetra. The span depends on care and genetics; what shortens it is unstable or poor water quality, chronically high temperatures kept up long-term, too small a group, and weak mass-bred stock.

Common mistakes

• Keeping too small a group. One to five fish become stressed, faded and hidden; buy eight to ten or more for a proper shoal.
• Adding them to a brand-new, uncycled tank. Hardy does not mean indestructible — new-tank instability drives most early losses.
• Expecting fry in the wrong water. They live happily in neutral, moderately hard water but will not breed outside soft, acidic conditions, so don't expect spawning in hard tap water.
• Buying the wrong fish by mistake. What is sold as 'harlequin' may actually be the slimmer espei (lambchop), the grey-and-orange hengeli (glowlight), or the newly-described Trigonostigma truncata — check for the broad, full triangular wedge and deep pink body to confirm a true heteromorpha.
• Housing them with predators or boisterous nippers. Adult angelfish, large cichlids and tiger barbs are wrong company for a small, peaceful shoaler.

Signs of trouble

• Colour fading and a fish dropping out of the shoal — usually water quality, an immature tank, or too small a group.
• Clamped fins, hiding and loss of appetite — general stress cues worth investigating before they progress.
• Flicking and scratching against decor — early ich (white spot), the most common disease in this otherwise robust fish.
• Skittish, washed-out behaviour in a small group — a husbandry signal that the school is too small rather than an illness.

Is this fish right for you?

Don't buy harlequin rasboras if you can't keep a group of eight to ten, if your tank is only just set up and not yet cycled, or if your community contains predators their size or runs at hard-water rift-cichlid conditions. Be aware that modern farmed stock is paler than wild fish, so pick the most colourful, active, well-shaped specimens and avoid any tank holding sick, curved or faded fish. And confirm you are actually buying heteromorpha rather than a mislabelled espei, hengeli or truncata.

Bringing one home

Float the bag to equalise temperature, then add tank water gradually over fifteen to twenty minutes before netting the fish into a mature, cycled tank and leaving the transport water behind. Soft-water blackwater fish resent sudden chemistry changes, so a slow drip and a settled tank matter more than for tougher species, and quarantining new arrivals protects the rest of the shoal.

Common questions

How many harlequin rasboras should I keep?

Six is the bare minimum; eight to ten or more is the real target. They are shoaling fish — small groups stay skittish and washed-out, while a larger school is confident, tightly-grouped and far more colourful.

What is the difference between a harlequin and a lambchop (espei) rasbora?

Look at the black mark and body colour. The true harlequin has a broad, full triangular wedge and the deepest pink body; the lambchop/espei has a thinner, hook-shaped mark on a brighter copper body; and the glowlight/hengeli has a thin mark on a grey body with an orange glow. Stores routinely mislabel them.

What tank size do harlequin rasboras need?

A 60 cm footprint (about 15 gallons) is the right target, with a 10-gallon as a tight floor for six only and 20 gallons better for a full school. The constraint is swimming room and group size, not waste — they are tiny and low-waste.

What are good tankmates for harlequin rasboras?

They are the classic community schooler. Small peaceful tetras, corydoras catfish, otocinclus, kuhli loaches, other small rasboras, danios and honey or dwarf gourami all suit them. Avoid large or predatory fish and boisterous nippers such as tiger barbs.

Are harlequin rasboras hard to breed?

Yes, fairly — they are often called notoriously difficult, almost entirely because of strict water needs. They will only spawn in soft, acidic, dim water around pH 6.0–6.4 and 28 °C, laying adhesive eggs on the underside of a broad leaf. In ordinary hard tap water they simply won't breed.

Sources & confidence

Sources & confidence (9 species)

These back the Harlequin Rasbora figures and the previewed tank mates above. Each figure is read from the TankStocking species database (v2026.06); below is the care reference behind it and how confident we are in that data. Confidence reflects the source quality, not whether any pairing is safe. Full source list and the welfare model are on the methodology page.

• Harlequin Rasbora Trigonostigma heteromorpha — Seriously Fish (seriouslyfish.com/species/trigonostigma-heteromorpha) high confidence
• Amano Shrimp Caridina multidentata — Aquarium Co-Op amano shrimp care; Aquadiction high confidence
• Black Neon Tetra Hyphessobrycon herbertaxelrodi — Seriously Fish / Aqua-Fish (Hyphessobrycon herbertaxelrodi) high confidence
• Black Phantom Tetra Hyphessobrycon megalopterus — Seriously Fish (Hyphessobrycon megalopterus) high confidence
• Bleeding Heart Tetra Hyphessobrycon erythrostigma — Seriously Fish (Hyphessobrycon erythrostigma) high confidence
• Bolivian Ram Mikrogeophagus altispinosus — Seriously Fish (seriouslyfish.com/species/mikrogeophagus-altispinosus) high confidence
• Brilliant Rasbora Rasbora einthovenii — Seriously Fish (seriouslyfish.com/species/rasbora-einthovenii) high confidence
• Bronze Corydoras Corydoras aeneus — Seriously Fish (seriouslyfish.com/species/corydoras-aeneus) high confidence
• Cardinal Tetra Paracheirodon axelrodi — Seriously Fish (seriouslyfish.com/species/paracheirodon-axelrodi) high confidence

Care-guide sources (7)

This guide synthesises the references below; where they disagree, the range and the disagreement are noted in the text above. The figures in the key-facts box are read from the TankStocking species database (v2026.06). Full welfare model on the methodology page.

• FishBase — Trigonostigma heteromorpha — authority (Duncker 1904), genus move (Kottelat & Witte 1999), family Danionidae, max 5.0 cm TL, temp 22-25 C, pH 5.0-7.0, dH 5-12, peat-swamp/forest-stream habitat, diet, IUCN LC, group 5+/min 60 cm, highly commercial
• Seriously Fish — Trigonostigma heteromorpha — synonym Rasbora heteromorpha, 35-45 mm SL, temp 21-28 C, pH 5.0-7.5, soft water, 60x30 cm tank, group 8-10, sexing incl. wedge-outline cue, leaf-underside breeding, distinctions vs espei/hengeli, mass-bred colour loss
• Wikipedia — Harlequin rasbora — etymology (harlequin costume; heteromorpha 'differently shaped'), SL 4-4.5 cm/max ~5.1 cm, blackwater habitat, breeding pH ~6.4 and ~28 C, wedge description, 5-8 yr lifespan, full breeding account (6-12 eggs, ~80-300, hatch ~18 h), Innes 1935 cover fact, IUCN LC
• Aquarium Co-Op — Harlequin & Espei (Lambchop) Rasboras — ~76 F, ~pH 7.2, soft water, 6 in a 10-gal floor, stress in smaller schools, omnivore diet, tankmates, harlequin ~2 in vs espei smaller, pinkish-brown vs brighter-orange wedge distinction, breeding needs warmer soft acidic water
• Seriously Fish — Trigonostigma espei (Lambchop Rasbora) — 'false harlequin' names, copper/red body + thin lambchop mark vs heteromorpha's broad triangle vs hengeli's grey+orange, max 25-30 mm, group 8-10 — the espei/hengeli ID distinction
• Tan (2020), Trigonostigma truncata description (Raffles Bulletin of Zoology) — formal split from the heteromorpha complex; wedge termed the 'axine'; in truncata the axine does not reach the caudal-fin base (vs heteromorpha where it does), shallower body, bluish-lilac sheen; found in wild-caught aquarium stock
• Fishkeeping News — New species of harlequin rasbora described — plain-language summary of T. truncata vs heteromorpha; advice that trade 'harlequins' may be misidentified

More on Harlequin Rasbora

Related guides on TankStocking — each scored by the same welfare engine as the planner.

This care guide is a sourced planning reference, not veterinary advice — individual fish, filtration and maintenance all matter. Cycle the tank, test your water, and observe your fish. How TankStocking works →