Cyanogaster is a genus of tiny characid tetras from blackwater tributaries of the Amazon basin, described from the Rio Negro drainage of Brazil and Colombia. The name, Greek for blue belly, points to the genus's most distinctive trait, a patch of iridescent blue pigment on the lower body that shows through an otherwise near-transparent, glass-like frame, an adaptation shared by several tannin-stained blackwater tetras that use translucency as camouflage against predators. The atlas covers both described species, Cyanogaster geisleri and Cyanogaster noctivaga, which rank among the smallest characiform fish known, with adults spanning roughly 0.5–1.5 in.
Because of their minute size and specialized blackwater habitat, Cyanogaster species are seen only sporadically in the aquarium trade, typically through importers who specialize in Rio Negro biotope fish alongside other micro-tetras and dwarf pencilfish. Where they are kept, the approach mirrors other soft, acidic blackwater nano-fish: very soft, tannin-tinted water, small live foods sized to their tiny mouths, and dense planting or leaf litter to replicate the flooded-forest habitat they come from. Both described species are currently assessed as Least Concern.