The common name alludes to the black triangular patch on the body, reminiscent of the patterns found on the costume of a harlequin.

This relationship between the pectoral and pelvic fins is seen in characins as well as cyprinids, and is a feature of all the ostariophysans (fishes possessing an auxiliary mechanism for detecting sound consisting of a set of internal bones called the Weberian ossicles).

These fishes are more slender in body shape than T. heteromorpha, and the black marking, instead of being approximately triangular, has a horizontal stripe which tapers towards the caudal peduncle, and is greatly thickened and extended downwards below the dorsal fin.

The harlequin rasbora is a native of Malaysia, Singapore, Sumatra and Borneo.As for the population distributed in southern Thailand, it has been replaced by the described (Trigonostigma truncata) in 2020.

It inhabits streams and other watercourses characterised principally by low mineral content, high concentrations of dissolved humic acids, a consequence of those waters flowing through peat swamp forests.

The lifespan of the harlequin rasbora has not been systematically determined, but individuals in the aquarium can be expected, with good care, to live for five to eight years.

Temperature for breeding should be 28 °C (82 °F), and the parent fish should be conditioned heavily with live foods such as Daphnia and mosquito larvae prior to the spawning attempt.

Once spawning is completed, the parent fishes should be removed from the breeding aquarium to prevent instances of egg eating, which may occur with this species.

Upon hatching, the fry are translucent, around three to four millimetres long, and remain attached to the leaf upon which the eggs were laid for a further 12 to 24 hours, during which the yolk sac is absorbed.

Once this process is completed, the fishes become free-swimming, and at this stage, require very fine sizes of food such as live infusoria for a period of seven to 14 days, after which the fry are able to feed upon newly hatched brine shrimp.