“A group of fish of the same species are called fish. Two or more species of fish are called ‘fishes’.”

Actually, I believe it is ALWAYS correct to use “fish” for a plural, without regard to whether they are all of the same species or not. But “fishes” MUST refer to different species. (If you need to make a point of the fact that you are listing various species, use “fishes”.)

Note that this is a more recent distinction. The KJV Bible (1611) speaks of “five loaves and two fishes” and is not referring to different species of fish.

http://www.amonline.net.au/fishes/what/fish.htm

http://crofsblogs.typepad.com/english/2005/02/fish_or_fishes.html

My aquarium contains three different fishes: guppies, platies, and swordtails.

The North Atlantic stock of Gadus morhua is estimated to contain several million fish.

Here are some examples:

Bob and I went fishing, and we caught 17 fish.

There are four or five fish swimming around in my fish tank.

not fishes

just like deer

its not deers

its just deer