Parastremma is a genus of characin (family Characidae) from northern South America, with the three atlas species - P. album, P. pulchrum, and P. sadina - recorded from Colombia and Brazil. At 3.5–7 in, these fish run considerably larger than the small tetras most aquarists associate with the family, suggesting a mid-water schooling or loosely social fish rather than a true nano species, though the atlas does not have confirmed detail on their specific habitat, diet, or behaviour beyond that size range and native distribution.
This is a genus with little presence in either the aquarium trade or the popular literature; data are sparse enough that the atlas prefers to say less here rather than guess at care requirements or wild behaviour that have not been well documented in sources available to it. The IUCN picture - two Data Deficient assessments and one Not Evaluated - fits a pattern common among mid-sized Neotropical characins from less-surveyed river systems: a genus that is not necessarily rare, but has simply not received the targeted field study needed for a confident conservation status.