Hemigrammus is a large genus of small characins — the classic "tetra" body plan — native to the rivers and streams of tropical South America, with the atlas's 62 species drawn mainly from the Amazon and Guiana Shield drainages: Brazil, Venezuela, Colombia, Peru, Guyana, Bolivia, Ecuador and French Guiana. Sizes run from 1 in up to 4.5 in, though most species stay well under 2 in. Like other Characidae, Hemigrammus fish are laterally compressed, silvery-bodied schoolers with an adipose fin, and the genus is defined mainly by fine details of scale and tooth arrangement rather than any single dramatic feature, which is part of why its exact boundaries have shifted repeatedly as ichthyologists have revised the wider tetra family tree.
This is one of the aquarium hobby's bread-and-butter tetra genera, supplying long-running community-tank staples such as the glowlight tetra (H. erythrozonus) and the firehead tetra (H. bleheri, a rummy-nose relative sold widely under that Portuguese-derived trade name). Most species are peaceful, easy-natured schooling fish that do well in soft, slightly acidic water reflecting their blackwater and clearwater stream origins, and they breed as open egg-scatterers with no parental care. The conservation picture is reassuring by comparison with many South American genera: the atlas tally is overwhelmingly Least Concern (42 of 62), with only a single Vulnerable listing and a handful of Data Deficient or unassessed species, consistent with a genus of small, fast-breeding, widely distributed fish.