Right on the button. From Wikipedia (which we know is infallible... )
Fish or fishes
Though often used interchangeably, these words actually mean different things. Fish is used either as singular noun or to describe a group of specimens from a single species. Fishes describes a group containing more than one species. Hence, as plurals, these words could be used thus:
* My aquarium contains three different fishes: guppies, platies, and swordtails.
* The North Atlantic stock of Gadus morhua is estimated to contain several million fish.