Fish can be singular or plural. Use it for one aquatic animal or when counting several individual animals.
- A child spotted one fish in the pond.
- We counted twelve fish beside the pier.
The plural does not automatically change to fishes when the animals come from different species. If you are simply counting individual animals, fish is still the usual choice. For example, an aquarium may contain fifty fish, even if they are not all the same kind.
Fishes is a less common plural. Writers use it mainly when emphasizing different species, kinds, or groups of fish. It appears especially often in scientific, ecological, and formal writing.
- Researchers studied the native fishes of the river basin.
- The book describes fishes from several different families.
In these examples, the focus is not simply the number of individual animals. It is the variety of species or groups being discussed.
Compare these choices: “The diver photographed six fish” counts six animals. “The region is home to many freshwater fishes” highlights the different kinds found there.
In everyday English, fish is much more common. Choose fishes when the distinction among species or kinds is important to your meaning.