A peaceful, copper-orange shoaling fish marked by a distinctive black triangular 'pork-chop' patch on the rear body. A long-lived, beautiful, beginner-friendly staple of planted community tanks.
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Southeast Asia (Malay Peninsula, Singapore, Sumatra, southern Thailand)
Origin
Old World
Climate
🌴 Tropical
Water type
💧 Freshwater
Family
Cyprinidae
Genus
Trigonostigma
Part of the Rasboras
Small, peaceful schooling cyprinids — from tiny nano species to mid-sized community fish — that show their best color and behavior in groups within soft, well-planted tanks.
From the minimum an animal needs to be kept humanely, up to the ideal setup. Bigger is almost always better — minimums are floors, not targets.
Photo coming soon
Minimum
Shoal planted tank
15 gal / 57 L planted
Trigonostigma heteromorpha reaches 2 in. 15-gal planted minimum for a shoal of 6+, with soft slightly acidic water, driftwood, plants, and gentle filtration. Peaceful classic community fish.
Recommended
Planted community shoal
20 gal / 76 L long planted
20-gal long planted with a shoal of 8–10, driftwood, leaf litter, and peaceful tankmates. Coppery orange colour with black triangle marking.
Tucsouffle / CC BY-SA 3.0 (Wikimedia Commons)
Photo coming soon
Ideal
SE Asian biotope
29 gal+ / 110 L+ biotope
SE Asian peat-swamp biotope with leaf litter, driftwood, very soft acidic water, dim lighting, and a shoal of 12+. Excellent colour and natural shoaling behaviour.
Life & growth stages
How this animal changes through its life — each stage often has its own care, diet and space needs.
Photo coming soon
Egg
Fish eggs are small, translucent spheres, often laid in clutches on plants, substrate, or in a nest — or carried/brooded by a parent in livebearing and mouth-brooding species. A dark eye spot and the curled embryo become visible inside as development progresses.
Photo coming soon
Fry
Newly hatched fry are tiny and semi-transparent, frequently still carrying a yolk sac that fuels them before they feed freely. They lack full fin structure and adult coloration, staying near cover until they can swim and forage on their own.
Photo coming soon
Juvenile
Juveniles look like miniature adults but with developing fins and muted or different markings; many species shift pattern and color as they mature. Growth is rapid at this stage given clean water and steady feeding.
Adult
Adults show the species' full size, finnage, and mature coloration, and are sexually mature. Many fish develop sex-specific differences in size, color, or fin shape, which can intensify during breeding.
Color & pattern variants
Natural variants occur in the wild; selectively bred (man-made) variants were developed in captivity.
A group is comfortable in a planted aquarium of at least 60 L (around 15 gallons), with larger tanks allowing bigger, more striking shoals. Recreate their Southeast Asian forest-stream habitat with a dark substrate, driftwood, leaf litter, dense planting, and subdued lighting, which makes their orange body and black wedge stand out.
They prefer soft to moderately hard, slightly acidic to neutral water with a pH around 5.5-7.0 and a temperature of roughly 22-26 C (72-79 F). Stable, well-filtered, clean water in a mature tank keeps them healthy; tannin-stained 'blackwater' conditions are ideal but not essential.
Substrate
A dark, fine substrate such as smooth sand or fine rounded gravel best displays their coppery-orange bodies and black triangle markings. A natural blackwater-style setup with botanicals and leaf litter mimics their soft, peaty native streams.
Equipment & setup
A gentle filter (sponge or low-flow hang-on-back) and a heater set to 73-82F suit these soft-water fish; aim for slightly acidic to neutral pH. Dim or moderate lighting filtered through floating plants brings out their best color and reduces shyness.
Diet
Harlequin rasboras are omnivores with small mouths. Provide quality flakes and micro-pellets as a staple, supplemented with small live and frozen foods such as daphnia, baby brine shrimp, cyclops, and bloodworms.
Feed modest amounts once or twice daily. A varied diet enhances their warm copper-orange coloration and conditions them well.
Behavior & temperament
This is a calm, gregarious shoaling species that should be kept in groups of at least 8-10 for the best color, confidence, and natural schooling behavior. They are entirely peaceful and occupy the middle of the tank.
They make excellent community fish with other small, gentle species such as small tetras, pygmy corydoras, dwarf gouramis, and shrimp. Heavy planting and dimmer lighting offer enrichment and security, encouraging them to display in the open.
Health
Harlequins are hardy and long-lived when well cared for, but like all small cyprinids they are vulnerable to ich, fin rot, and bacterial infections triggered by poor water quality or chilling. Tank-bred stock is generally robust.
Quarantine new fish, acclimate slowly, and keep parameters stable. Their longevity, often well beyond five years, depends on consistent maintenance, an adequate shoal, and a mature, stress-free aquarium.
Tips, DIY & hacks
Keep them in a shoal of at least 8-10 so they school tightly and stay confident. Add Indian almond (catappa) leaves to soften and tint the water cheaply, which also enriches the tank and encourages natural behavior and spawning.
