The plural of "fish" is "fish". This is because "fish" can be both singular and plural depending on the context.
* Singular: "I caught a big fish."
* Plural: "There were many fish in the lake."
You can use the word "fishes" when referring to different species of fish, but it's not commonly used. For example, "There are many different fishes in the ocean."
However, "fishes" is more often used in a scientific context when referring to multiple species of fish.