Otocinclus is a genus of dwarf suckermouth catfish in the family Loricariidae, native to South America, with the atlas's records spanning Brazil, Peru, Colombia, Argentina, Bolivia, Paraguay, Uruguay, and Ecuador. Like other loricariids, otos have an armored body and a ventral sucker-mouth, but at an adult size of roughly 1–2 in across the genus's 20 atlas species, they are far smaller than most of their relatives -- among the smallest members of the family kept in the aquarium hobby. They are peaceful, schooling algae- and biofilm-grazers, typically found clinging to submerged wood, leaves, and rocks in the slow-moving margins of streams and rivers, where soft algae and diatoms make up the bulk of their diet.
Otocinclus is one of the most widely kept algae-control fish in the planted-aquarium hobby, valued for its small size, peaceful temperament, and constant grazing on soft algae and biofilm that few other fish will touch. Otocinclus affinis (Golden Otocinclus) and O. vittatus (Common Otocinclus) are the species most often seen in the trade. Because the great majority of otos sold have historically been wild-caught, they have a long-standing hobby reputation for needing a well-established, algae-bearing tank and a careful, slow acclimation before they settle in and feed reliably, since fish pulled straight from a low-food wild environment into a bare new tank commonly struggle. Conservation status is generally favorable, with most assessed species Least Concern, though it includes one Endangered and a couple of Data Deficient species.