Fish / Fishes

Is 'fish' or 'fishes' the correct plural form of the word 'fish'? Dictionaries list both as correct but I have always believed and taught that the plural of 'fish' is 'fish'.
Thanks for you help.

Fish is both singular and plural within a species. One salmon in a fish, three salmons are 3 fish. Fishes is the plural when you have more than one species. One salmon is a fish, but one salmon and one flounder are 2 fishes.

I didn't make this up (how could I).

Guest wrote:

Fish is both singular and plural within a species. One salmon in a fish, three salmons are 3 fish. Fishes is the plural when you have more than one species. One salmon is a fish, but one salmon and one flounder are 2 fishes. I didn't make this up (how could I).

You're right!!!!!!!!!!!!!!!!!!!