Gymnocorymbus is a small genus of characins (family Characidae) native to South America, recorded in the atlas from Bolivia, Brazil, Colombia, Argentina, Guyana, Peru, Trinidad and Tobago and Venezuela. Members of the genus share a deep, laterally compressed, almost diamond-shaped body and a dusky to black patch through the caudal peduncle and rear fins, a pattern that gives the group its common look on sight even across different species. The atlas covers 4 species, from about 2–3 in, occupying slow-moving streams, floodplain channels and blackwater habitats across a broad swath of the continent's lowlands.
The genus is best known in the hobby through Gymnocorymbus ternetzi, the black skirt tetra — one of the longest-established and most widely bred aquarium fish in the trade, prized for its flowing black fins and easy temperament, and the source of long-fin and colour-morph varieties sold worldwide. Gymnocorymbus thayeri (Thayer's tetra) and Gymnocorymbus bondi (silver tetra) are seen far less often but share the same easygoing, schooling nature: peaceful, adaptable community fish that do best in groups and are not fussy about water chemistry within normal tropical ranges. Conservation status is not a concern for the well-studied members of the genus, though the atlas tally of three Least Concern against one Not Evaluated reflects that not every species has had a formal assessment.