Harlequin Rasbora (Trigonostigma heteromorpha)

The Harlequin Rasbora (Trigonostigma heteromorpha) is native to the blackwater rivers, streams, and peat swamps of peninsular Malaysia, Thailand, and Singapore, as well as the Indonesian islands of Sumatra and Borneo. Since its introduction to the hobby in the 1930s, it has remained one of the most consistently popular aquarium fish, prized for its combination of striking appearance, peaceful temperament, and ease of care.

The distinctive coloration pattern is unique: the posterior half of the body displays a bold black triangular or wedge-shaped patch — called the 'lambda mark' — against a warm copper-red to orange-red body. The anterior body shimmers with gold iridescence. The black patch is more sharply defined in males, while females show a slightly rounded bottom edge to the patch. Males are slimmer; females are more robust.

In their natural blackwater habitats, Harlequin Rasboras inhabit heavily vegetated, soft, acidic waters with minimal mineral content and significant tannin staining. Despite these original conditions, they have adapted well to a range of aquarium water conditions over generations of captive breeding.

Harlequin Rasboras form tight, cohesive schools in mid-water, making them visually spectacular in groups of 10 or more. Their schooling behavior, striking coloration, and compatibility with essentially all peaceful community species make them a perennial classic of the hobby.

Harlequin Rasboras adapt well to pH 5.5–7.5, hardness 1–12 dGH, and temperatures 22–28°C. While originally blackwater fish, commercially bred specimens readily accept moderate water conditions. However, soft, slightly acidic water with tannin content produces the most vivid coloration.

A minimum 60-liter planted aquarium suits a group of 8–10. Provide a dark substrate, driftwood, dried Indian almond leaves, and dense fine-leaved plantings. Moderate gentle current replicates their natural stream flow. Open mid-water swimming space allows natural schooling behavior.

Feed quality flake food, micro pellets, and regular live or frozen supplements: daphnia, baby brine shrimp, cyclops, and small bloodworms. They are enthusiastic, easy feeders that accept virtually all standard aquarium foods.

Harlequin Rasboras are entirely peaceful and one of the safest choices for mixed community aquariums. Compatible with tetras, other rasboras, corydoras, small catfish, gouramis, livebearers, and peaceful dwarf cichlids. Avoid very large or aggressive species.

Keep in groups of at least 8–10. They look spectacular in groups of 20+ where the synchronized schooling behavior and collective copper-red flash of the lambda marks creates an extraordinary display. Weekly water changes of 25–30% maintain water quality.