Trigonostigma heteromorpha
Bulletproof beginner schooler. The black wedge marking is iconic. Pairs well with rummynose tetras and corydoras in a community planted tank.
Harlequin Rasbora (Trigonostigma heteromorpha) reaches 4–5 cm as an adult and needs a minimum tank of 60 L. Native to Thailand, Malaysia, Sumatra, it lives in the mid water column with a peaceful temperament. Aim for 22–28 °C, pH 5.5–7.5, and 2–12 dGH hardness. Lifespan is 5–8 years with good care. Keep harlequin rasbora in groups of 8+, yes schoolers need numbers to display natural behaviour. Diet: omnivore, Flake, micro pellets, frozen bloodworms, daphnia. Plant-safe: Yes. Shrimp-safe: Mostly (may eat shrimplets).
Care at a glance
Bulletproof beginner schooler. The black wedge marking is iconic. Pairs well with rummynose tetras and corydoras in a community planted tank.
By Mike ElmiraUpdated 2 min read
Part of our complete guide to aquarium fish for the planted tank.
The parameters that decide whether harlequin rasbora fits in your tank.
Mid
Yes
Group of 8+
Cyprinidae
Omnivore
Flake, micro pellets, frozen bloodworms, daphnia.
5–8 yrs
Medium
Slow tropical streams and peat swamps of Sumatra
Thailand, Malaysia, Sumatra
Tank-mate safety and the species this one is documented to thrive (or fail) alongside.
Cardinal and ember tetras, corydoras, otocinclus, sparkling gouramis, German blue rams (similar parameters), hillstream loaches (different layer).
Aggressive cichlids, fin-nippers, very small nano fish (harlequins can intimidate chilis).
Hard-won lessons from the tank.
Bulletproof beginner schooler — but they truly shine in a soft-water blackwater tank where the wedge contrasts against dark surroundings. A 'community workhorse' that looks stunning when treated like a specialty fish.
'Trigonostigma' = 'triangle mark'. 'Heteromorpha' = 'different form' (referring to its unusual deep body vs. other rasboras).
What can go wrong and how to spot it.
Failure modes, in order of how dramatic the fix is.
Robust. Watch for parasites in wild-caught individuals; almost all stock is now captive-bred.
Often confused with lambchop rasbora (T. espei) and porkchop rasbora (T. hengeli) — both have smaller wedges and are slimmer-bodied. Heteromorpha has the boldest, biggest wedge.
The practical routine, read top to bottom.
60 L upwards. Planted with crypts and floating plants. Sand or fine gravel. Soft to moderate flow. Moderate lighting.
2–3 weeks standard.
Where it comes from, how it behaves, and the variants you'll see at retail.
Where it lived before it came home.
Peat swamps and slow streams of Malay Peninsula, Sumatra, southern Thailand. Soft to moderately hard, neutral water; in the wild often very acidic blackwater.
Small worms, insects, larvae, plant matter.
Not threatened; captive-bred globally.
How they pair, reproduce, and grow.
Males are slimmer with the black wedge having a slightly extended bottom edge. Females are rounder, wedge slightly shorter and rounded.
Unusual among rasboras — egg-layers that attach eggs to the underside of broad plant leaves (especially Cryptocoryne). Soft acidic water and 26–28 °C. Adults will not typically eat eggs but should be removed after spawning.
The named cultivars and the lookalikes worth flagging.
Wild type + a less common 'gold' variant (T. heteromorpha 'Gold').
Direct answers to the questions search engines and AI assistants surface most often about this species.
Harlequin Rasbora (Trigonostigma heteromorpha) needs a minimum tank of 60 L. They live in the mid water column and should be kept in groups of 8+, so a longer footprint matters more than depth.
Target 22–28 °C, pH 5.5–7.5, and 2–12 dGH hardness. Acclimate slowly when moving them between water sources.
Shrimp safety: Mostly (may eat shrimplets). Plant safety: Yes.
Harlequin Rasbora are omnivore. Flake, micro pellets, frozen bloodworms, daphnia.
On Fin & Stem's 1–5 difficulty scale this species rates 1/5. Almost unkillable, a solid first-tank choice. Breeding difficulty: medium.
Typical lifespan in a well-maintained tank is 5–8 years.
A planted tank is a system. Pair this fish with one entry from each other pillar to plan the whole scape.
