Otocinclus affinis, or Otto for short.  There are at least three varieties of Otocinclus that look and behave similarly (Ottocinclus affinis, arnoldi, and vittatus).  I have had experience only with O. affinis, which is from southeastern Brazil, and never gets much longer than about two inches.  All Ottos are terrific algae eaters, eating algae which is scraped off leaves, rocks and the sides of the aquarium. Ottos rapidly clean up brown algae (some refer to it as diatomacious algae), and will also eat green algae.  (Ottos will not touch red or brush algae. The Siamese algae eater is the only known fish to eat red algae.) One of these fish per ten gallons of tank water will significantly help with algae control.  Unlike other types of sucker mouth catfish, Ottos are nearly always good natured, and never become territorial.  O. affinis prefers temperatures of between 70 and 85 degrees Fahrenheit.  It can tolerate slightly acid to slightly alkaline water without difficulty, though some references indicate slightly acid is best.