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Black Harlequin Rasbora
Trigonostigma heteromorpha
Also known as Black Harlequin, Dark Harlequin Rasbora
Cyprinidae
📍 Southeast Asia, including Thailand, Malaysia, Singapore, and Sumatra; found in slow-moving, peat-stained forest streams and pools
The Black Harlequin Rasbora is a selectively bred or naturally occurring dark variant of the classic Harlequin Rasbora, displaying a deep black or very dark body with the characteristic triangular black patch on the posterior half, sometimes with a subtle iridescent sheen. It is a peaceful, active schooling fish that adds striking contrast to planted aquariums and thrives as part of a community setup.
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Care Guide
Diet
The Black Harlequin Rasbora is an omnivore that readily accepts high-quality micro pellets and flake foods as a staple diet. Supplementing with small live or frozen foods such as daphnia, brine shrimp, and micro worms will enhance coloration and overall health. Feed small amounts two to three times daily, offering only what the fish can consume within two minutes to maintain water quality.
Behavior
Black Harlequin Rasboras are active, sociable schooling fish that feel most secure and display their best behavior when kept in groups of six or more. They occupy the middle water column and tend to swim in loose, coordinated groups, making them a visually dynamic centerpiece for community tanks. They are non-aggressive and will not harass tank mates, though they may become timid or stressed if kept in too-small groups or with boisterous species.
Breeding
Breeding is moderately challenging and is best triggered by soft, slightly acidic water with a pH around 5.5-6.5 and temperatures near the upper end of their range. Like other Trigonostigma species, they are egg scatterers that deposit adhesive eggs on the undersides of broad-leaved plants such as Cryptocoryne or Anubias. Parents should be removed after spawning as they will consume the eggs; fry hatch within 24-36 hours and require infusoria or commercial fry foods until large enough to accept baby brine shrimp.
Tank Mates
Similar size, water parameter requirements, and peaceful temperament make them ideal companions in a planted community tank.
Bottom-dwelling, peaceful, and shares a preference for soft, slightly acidic water; occupies a different swim zone with no competition.
Generally peaceful and compatible in size and water parameters; monitor for any mild aggression from the gourami toward smaller fish.
Tiny, peaceful nano fish that thrives in similar soft, acidic water conditions and complements the Harlequin's schooling behavior.
Common Diseases
Ich (White Spot Disease)
Fine white spots resembling grains of salt on the body and fins, flashing or rubbing against surfaces, lethargy, and loss of appetite.
Gradually raise water temperature to 28-30 C (82-86 F) to accelerate the parasite's life cycle, and treat with a copper-based or malachite green medication according to label directions; perform partial water changes throughout treatment.
Velvet Disease (Oodinium)
Gold or rust-colored dust-like coating on the skin, clamped fins, rapid gill movement, lethargy, and loss of appetite.
Dim tank lighting, raise temperature slightly, and treat with a copper-based medication; quarantine affected fish and perform regular water changes to reduce parasite load.
Fin Rot
Fraying, discoloration, or progressive deterioration of fin edges, sometimes accompanied by redness or inflammation at the fin base.
Improve water quality through frequent partial water changes, identify and correct the underlying cause such as poor water conditions or aggression, and treat with an appropriate antibacterial medication such as kanamycin or erythromycin if bacterial infection is confirmed.
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Quick Facts
- diet
- Omnivore – small live, frozen, or quality dry foods
- lifespan
- 3-5 years
- max size
- 4.5 cm (1.75 in)
- tank size
- 10 gallons minimum
Water it likes
- ph
- 5.5-7.5
- ammonia
- 0 ppm
- nitrate
- <20 ppm
- hardness
- 1-10 dGH
- temperature
- 72–81°F (22–27°C)
Legality
No state or federal restrictions on record for this species.
Not legal advice, and possibly incomplete or out of date. Rules vary by state and locality and change over time — always confirm the current regulations with your state wildlife or agriculture agency before buying, keeping, or shipping this species.