Some nouns have the same form in the singular and the plural and the word `Fish` is such an example. As such, fish is the usual plural word for fish whereby the plural form is the same as the singular.
However, strictly speaking, fish is more correctly used to refer or describe an individual fish or the plural of a single species of fish (multiple individuals of the same species). The term `fishes` on the other hand, is used to refer or describe the plural of different species of fish (more than one species of fish).
So, Fish can be used in a singular form when describing one fish, as well as, in a plural form only when describing two or more fish of the same species. Examples:
- A fish is a cold blooded vertebrate animal that lives in water. (singular)
- Salmon is a fish. (singular)
- I swam with a school of 200 fish. (plural)
- Fish are cold-blooded vertebrate animal that live in the water. (plural)
- I caught six fish in the river yesterday. (plural)
While Fishes is always plural and is used when describing more than one species of fish.
- There are a lot fishes in the sea. (plural)
- Ichthyology is the study of fishes. (plural)
Hence, if you are speaking or writing about one or more than one fish of the` same species (one type of fish) you then use the word `fish` and likewise, if you are speaking or writing about more than one species of fish (different types of fish) then you use the word `fishes`.