Harlequin rasbora

The harlequin rasbora is one of the most enduringly popular community fish in the hobby, cherished for its distinctive black triangular marking that tapers along the rear half of the body. Native to blackwater streams and peat swamp forests in Thailand, Malaysia, Singapore, and Indonesia, this small cyprinid reaches only 3.5–4.5 cm. Wild populations vary geographically — fish from southern Thailand and Johor tend to be smaller and slimmer than those from Singapore and the Sunda Islands, which are deeper-bodied and more richly coloured.

Harlequin rasboras are micropredators that feed on small insects, worms, crustaceans, and zooplankton in nature. In the aquarium they are easy to feed but show their best colours when offered a varied diet of quality dried foods supplemented with small live and frozen fare such as Daphnia, Artemia, and bloodworm. The tank should be well-furnished with a dark substrate, driftwood, and patches of shade, with floating plants to diffuse the light. An aquarium with base dimensions of at least 60 cm x 30 cm (~54 litres) is recommended, as this species must be kept in a group.

The harlequin rasbora is exceptionally peaceful and an ideal resident of the well-maintained community aquarium. It mixes well with similarly small, peaceful species including other rasboras, small tetras, Corydoras catfish, and dwarf cichlids. Harlequins are shoaling fish and should be maintained in groups of at least eight to ten specimens to feel secure and display their natural behaviour. They thrive in soft, acidic water (pH 5.0–7.5) across a temperature range of 21–28 °C.