Species · Tetras & rasboras
Harlequin Rasbora
Trigonostigma heteromorpha
One of the best first schooling fish there is — hardy, peaceful, and gorgeous in a group. Keep a real shoal in soft-to-neutral water and it just thrives.
Difficulty: Beginner Tropical community tank ✓ Fact-checked care guide
Quick facts
| Adult size | ~4.5 cm / 1.8 in |
|---|---|
| Minimum tank | ~40 L / 10 US gal for a shoal — SeriouslyFish recommends a 60 × 30 cm base |
| Temperature | 23–27 °C / 73–81 °F (within the species range of 21–28 °C / 70–82 °F) |
| pH | 5.0–7.5 (soft, acidic to neutral) |
| Hardness | 1–12 dGH (soft water) |
| Filtration & flow | Cycled, gentle-to-moderate filtration |
| Diet | Micro-omnivore — small flake/micro-pellet plus tiny live/frozen foods |
| Social | Shoaling — keep at least 8–10; the more, the better |
| Temperament | Very peaceful |
| Lifespan | 5–6 years |
Overview
The harlequin rasbora (Trigonostigma heteromorpha) is a hardy, peaceful copper-and-black schooler that has been a hobby staple for over a century — and one of the very best schooling fish for a first community tank.
It is genuinely easy and long-lived, asking only for a proper group and reasonably soft, clean water. Mass-bred stock can be a little less colourful than wild fish, but it is still tough and rewarding.
💷 Costing it out? See how much a harlequin rasbora really costs to keep — the harlequin rasbora itself is usually the cheapest part.
Water & tank
Harlequins come from soft, often peat-stained forest waters, so they prefer soft, slightly acidic-to-neutral water (pH 5.0–7.5, 1–12 dGH) — though captive-bred fish are adaptable. They colour up beautifully in a planted, gently-lit tank.
Give a shoal at least ~40 L / 10 US gal (SeriouslyFish recommends a 60 × 30 cm base), cycled and planted.
Feeding
A micro-omnivore with a small mouth — a good small-grade flake or micro-pellet base plus tiny live/frozen foods (daphnia, baby brine shrimp) for colour and condition.
Feed small amounts once or twice a day.
Behaviour & tankmates
An ideal peaceful community fish — perfect with other small, calm species (other rasboras, small tetras, corydoras, dwarf shrimp with cover). Avoid fin-nippers and large or aggressive tankmates.
Keep a real group of 8–10+ — in small numbers they are shy and washed-out; in a proper shoal they are confident and constantly out in the open.
What to expect
A new group may be shy for a day or two, then settle into loose, cheerful schooling. Deep colour and active shoaling are your "all is well" signs.
Myths, kindly busted
“A few rasboras are enough.”
They are shoaling fish — keep 8–10 or more; small numbers are stressed, pale and reclusive.
“They need pristine soft water or they die.”
They prefer soft water but captive-bred fish are adaptable and hardy — stable, clean water matters more than a perfect number.
🐟 Did you know? — Harlequin Rasbora fun facts
Little true delights about the harlequin rasbora — each one checked against a named source, no pet-shop myths.
- Unlike most small cyprinids that scatter eggs, harlequins spawn upside-down, gluing their eggs to the underside of a broad plant leaf. SeriouslyFish
- The harlequin was first exported for the hobby in the early 1900s, making it one of the trade's most enduring species. SeriouslyFish
- Its trademark black “wedge” can be sexed by shape: sharp and well-defined in males, but softly rounded in females. SeriouslyFish
When to act
We triage, not diagnose — but these are worth a closer look (starting with a water test):
- Persistent hiding, clamped fins or fading colour across the group — test the water and check the shoal is big enough.
- White spots or rapid breathing — test water quality and act early.
Heading to a vet? Print the Harlequin Rasbora vet-prep sheet — what to bring, what to tell them, and the questions to ask.
Sources & further reading
- SeriouslyFish — Trigonostigma heteromorpha — Adult ~45 mm SL; recommended 60 × 30 cm base; temperature 21–28 °C / 70–82 °F; pH 5.0–7.5; hardness 1–12 dGH (soft); very peaceful shoaling community fish.
Written & fact-checked by The Finfern Team against the sources above — kind, general guidance, not a diagnosis. If something looks wrong, test your water and, if it persists, contact a fish-savvy vet.
Get the free Harlequin Rasbora Starter Kit
The essentials at a glance, a setup checklist and a first-30-days plan for your harlequin rasbora — what to get right and what to watch for. Join the shoal and we’ll send it over. Free sheets and tools always stay open, no gatekeeping.
Water-first tips and new guides the moment they land. No spam — unsubscribe anytime. See how we handle your data in our privacy notice.