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Harlequin Rasbora

Trigonostigma heteromorpha

The Copper Wedge (1.5 - 2 inches / 4-5 cm). The Harlequin Rasbora is a legendary, bulletproof beginner schooling fish. Instantly recognizable by its deep, metallic orange/copper body stamped with a massive, pitch-black triangular 'pork-chop' or wedge shape on its back half. They are famous for being incredibly tight schoolers, moving together in a highly synchronized, stunning flash of copper. Unlike Tetras or Barbs, they are 100% safe to keep with Bettas.

Taxonomy
Class:Actinopteri
Order:Cypriniformes
Family:Cyprinidae
Subfamily:Rasborinae
Genus:Trigonostigma
Species:Trigonostigma heteromorpha

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Water Temperature

22 - 28 °C

pH Value

5 - 7.5

Water type

Freshwater

Tank level

Middle

Adult size

5 cm

Minimum tank volume

>= 54 L

Minimum group

>= 8

GH

1 - 12 dGH

KH

1 - 8 dKH

TDS

25 - 260 ppm

Conductivity

39 - 406 uS/cm

Species description

Geographical Origin and Habitat: Distributed in Malaysia, Singapore, Sumatra, and southern Thailand. Inhabits slow-flowing forest streams and peat swamps (blackwater) shaded by the dense jungle canopy, where the water is heavily amber-tinted due to decaying fallen leaves and very poor in minerals.
Taxonomy and Morphology: Cyprinid with a stocky and deep body (compared to the more slender T. espei). Reaches 4-5 cm (2 inches). The name "heteromorpha" (different shape) alludes to the singular body shape and the huge bright black triangular wedge covering the rear half of the flanks, serving as a visual signal for the school in dark waters.
Social Behavior: Extremely gregarious and peaceful. Indispensable to keep them in large schools (at least 10-15 specimens). Swimming is elegant and compact. They spend the day swimming in tight swarms in the middle water column, offering one of the most beautiful visual displays in a well-planted community aquarium.
Coloration and Sexual Dimorphism: Base coloration pink-salmon or silvery copper, dominated by the large black rear wedge. Males have a much more intense copper-red color, and the lower edge of their black triangle extends pointed forward and downward (on the belly). Females are paler, bulkier, and the lower edge of their triangle is straight.

Care, breeding and tankmates

Aquarium Setup: Aquarium of at least 60-80 cm (24-30 inches). They require subdued lighting (use floating plants like Amazon Frogbit) and a dark setup with driftwood, roots, and dark sandy substrate to make their livery stand out. Insert broad-leaved Cryptocoryne bushes, essential to provide them with necessary breeding substrates.
Diet and Feeding: Micropredator of the surface and mid-water. Feeds on insects, worms, and planktonic crustaceans. In captivity, it readily accepts crushed high-quality flakes, but needs regular feedings of live/frozen food (Daphnia, Brine Shrimp, Bloodworms) to stimulate colors and breeding activity.
Water Quality: Very adaptable fish thanks to years of commercial breeding, but to thrive and breed it needs soft, acidic water. Temperature: 22-28°C (72-82°F). Ideal pH between 5.0 and 7.0 (max 7.5). Hardness (GH) between 1.0 and 12.0. Excellent tolerance if water is kept hygienically impeccable through good filtration.
Compatibility and Cohabitation: Perfect community fish. Ideal tankmate for Asian Gouramis (Trichogaster, Sphaerichthys), Bettas, Corydoras, and Loaches. Absolutely avoid pairing with aggressive or very large fish that might intimidate or prey on them, forcing them into perpetual hiding.
Aquarium Breeding: Fascinating but difficult in common tanks. Unlike other rasboras, T. heteromorpha lays adhesive eggs by swimming upside down under broad leaves of aquatic plants (Cryptocoryne). Breeders must be separated after spawning. Vital requirement for hatching: extremely acidic and soft water (pH 5.5, GH < 2).
Risks and Diseases: Suffers terribly from stress due to small numbers (fewer than 6 specimens) or overly bright tanks, which cause chronic fading, refusal to eat, and rapid immune decline with the risk of Ich (Ichthyophthirius).

Geographic Origin and Distribution

Distributed in Malaysia, Singapore, Sumatra, and southern Thailand. Inhabits slow-flowing forest streams and peat swamps (blackwater) shaded by the dense jungle canopy, where the water is heavily amber-tinted due to decaying fallen leaves and very poor in minerals.

Origin

Selective breeding and cultivarsNorth AmericaSouth and Southeast AsiaCosmopolitan or introduced

Fish profile

Temperament
Semi-aggressive
They are peaceful and somewhat timid if kept in numbers that are too small. A school of at least eight to ten individuals brings out their best behavior. In a properly sized group, they exhibit fascinating social dynamics and vibrant colors.
Diet
Omnivore
They are micropredators that feed on tiny insects in nature. In the aquarium, they readily accept high-quality micro-pellets and crushed flakes. Supplementing their diet with live or frozen daphnia and cyclops ensures optimal health and coloration.
Sex ratio
School
A mixed group works well, though more females can reduce male competition. Males are generally slimmer and more intensely colored during the breeding season. Having a large enough school ensures any minor aggression is spread out evenly.
Feeding frequency
Twice daily
It is best to feed them small amounts two or three times daily. Their small stomachs require frequent but tiny meals to prevent bloating. Removing uneaten food after a few minutes helps maintain excellent water quality.
Bioload
This species produces very little bioload due to its small size. Weekly water changes and standard filtration are adequate to keep parameters pristine. Still, their schooling nature means the total bioload of the group must be considered.
Flow
Stagnant
They come from slow-moving rivers and shaded forest streams. Gentle to moderate water circulation mimics their natural environment perfectly. Strong flow from oversized pumps can easily exhaust these delicate swimmers.
Reproduction
Oviparous
Breeding is a challenging but rewarding endeavor for advanced hobbyists. They are egg-scatterers that lay adhesive eggs on the undersides of broad leaves. The parents provide no care and should be removed after spawning to prevent egg cannibalism.
Compatibility & tankmates
They are exceptionally peaceful and make great community fish. They should never be kept with large or aggressive tank mates that might eat them. Ideal companions include small tetras, peaceful bottom dwellers, and dwarf shrimp.
Tank level
Middle
They predominantly occupy the middle to upper levels of the water column. They enjoy open swimming spaces surrounded by tall aquatic plants. Rarely do they venture to the substrate unless chasing a sinking morsel of food.

Window to the official scientific data of the species

Data synced on: 06/15/2026

Scientific Parameters & Models

Trophic level
3.3 ± 0.4 se
Resilience
High (less than 15 months)
Phylogenetic uniqueness (PD50)
0.5312
Bayesian Length-Weight
a = 0.01995, b = 3.01
VulnerabilityLow vulnerability (10 / 100)

Image gallery

Licensed images linked to the species or, when marked, to the closest representative taxon.

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Last updated: 06/30/2026
Harlequin Rasbora (Trigonostigma heteromorpha) Care Guide & Tank Setup | Atlarium