The term “fish” is used when referring to one species of fish on its own. For example 10 tunas can be called 10 fish.
On the other hand the term “fishes” is used when referring to multiple species of fish. For example 10 tunas, 5 clown fish and 2 butterfly fish are known as 17 fishes.
Fishes with an apostrophe is used for the plural possessive of the fish. The fishes’ fins were all pink, meaning the 10 tunas, 5 clown fish and 2 butterfly fish all have pink fins.