An otocinclus can grow to be two inches long when fully mature. Most otocinclus stay between one and two inches long. Even the largest species, Otocinclus flexilis, only reaches 2.2 inches when fully grown.
Their small size makes otocinclus one of the best choices for fishkeepers with limited space. These fish don’t need much space to thrive.
As a general rule, an inch of fish requires one gallon of swimming space. This means an otocinclus only needs two gallons of space. A small group of five to six otocinclus can live happily in a small 10-gallon species tank.