Dictionary of Ichthyology  

doctor fish = 1) any of a series of unrelated fish species that are supposedly helpful to other fishes, e.g. tench (Tinca tinca, Cyprinidae) slime is said to cure wounds; Garra rufa and Cyprinion macrostomum (Cyprinidae) in Turkish hot springs in the Kangal area clean dead skin fragments from humans with psoriasis; the circumpolar Gymnelus viridis or fish doctor may be a cleaner fish but the reason for its name is unknown.

Garra rufa from Wikimedia Commons: http://commons.wikimedia.org/wiki/File:Doctor_fish.jpg 

Canadian Museum of Nature