Here's the breakdown:
* Fish can be both singular and plural.
* Singular: "There's a fish in the tank."
* Plural: "There are many fish in the ocean."
* Fishes is used when referring to different kinds or species of fish.
* "The aquarium has a variety of fishes, including goldfish, angelfish, and sharks."
So, in most cases, "fish" will be the correct plural form. "Fishes" is a more specific term used when discussing different types of fish.