When do you use fishes instead of fish?

"Fishes" is rarely used in modern English except in certain specific contexts, such as when referring to multiple species of fish or when discussing fish as a group for scientific or academic purposes. For example:

- "There are many different species of fishes that live in the ocean."

- "The study of fishes is called ichthyology."

In general, "fish" is the more common and versatile word to use when referring to a single fish or multiple fish.