See http://en.wikipedia.org/wiki/English_plural
... generally used as a plural when in the context of food, but forming a regular plural otherwise (thus "three lots of fish and chips", "the industry landed 5,200 tonnes of fish in 1998" but "the order of fishes", "the miracle of the loaves and fishes" the phrase "sleep with the fishes"); usage does vary, however, so that for example the phrase "five fish in an aquarium" might to another native user be "five fishes in an aquarium".



