Dictionary of Ichthyology  

fish lice (singular louse) = parasitic crustaceans on marine and freshwater fishes. The common ectoparasite is Argulus (Branchiura) which can move around on its host, causing several wounds. May result in lethargy, loss of appetite, anaemia through blood loss, osmotic stress and secondary infections. The effects of irritation are restlessness, darting, jumping, scratching and overproduction of mucus. Various chemical treatments of aquarium water will remove the parasite although they are often large enough to be picked off with tweezers as long as a topical antiseptic is applied to the wound site.
Caligus rapax, a copepod fish louse. U.S. Fish and Wildlife Service. Photograph by Brian W. Coad.

Canadian Museum of Nature