* "Fish" is generally used as both singular and plural. So, you would say "one fish" and "many fish".
* "Fishes" is sometimes used to refer to different species of fish. For example, "tropical fishes" or "aquarium fishes." However, even in these cases, "fish" is usually preferred.
* "Fishes" is also occasionally used for a scientific context when referring to a taxonomic group. For example, a researcher might say "the fishes of the Amazon River."
In most cases, "fish" is the correct and more common choice.