Trigonostigma is a genus of small rasboras in family Danionidae, native to Thailand, Malaysia, Indonesia, Cambodia, Singapore, and Vietnam. Species typically inhabit slow-moving, often blackwater streams, swamps, and peat forests, and most share a distinctive triangular black wedge marking on the body that gives the genus its common identity among aquarists.
The harlequin rasbora (Trigonostigma heteromorpha) is one of the most recognizable and widely kept schooling fish in the freshwater hobby, and its close relatives - the lambchop rasbora (T. espei), the glowlight rasbora (T. hengeli), and Somphong's rasbora (T. somphongsi) - are popular with nano and planted-tank keepers for similar reasons: small adult size (roughly 1–2 in across the genus), peaceful shoaling behavior, and good color under soft, tannin-stained water. Conservation status varies sharply within the genus - three atlas species are Least Concern, but one is assessed Critically Endangered and is flagged for CARES conservation priority, underscoring how localized some of these rasboras' ranges can be.