'Fish' is also the plural of the same type of fish, however when there are different types of fish in a water body, one could then call them 'fishes'.
What Is The Difference Between Fish And Fishes?
Fish is a word that can act as a noun or a verb. Example: I eat fish (noun). I like to fish (verb). Fishes is not the plural of fish. Fishes is the third person present form of the verb. He fishes the waters.