The word “fish” has a strange feature: while the word is singular and plural, there is an alternative plural, “fishes,” which is used to refer to more than one type of fish. So, “I own a fish,” “I own lots of fish,” and “I own fishes” can all be correct. Thus “all the fish in the sea” means each and every fish, while “all the fishes in the sea” means all the different types of fish.
English has a bunch of words where the singular and plural are the same (especially animal words, like “moose,” though I’ve never read a thorough explanation for that).Other examples are “cannon,” “species,” and “Euro”* (as in the currency; “bitcoin” is also used as a plural.)
One note about the Euro is that as I understand it, the intention was for “Euro” to be the plural, and that is the official position of the EU, but there is widespread use of “Euros” in languages where plurals are ordinarily formed by adding an “s.” There’s even a Wikipedia entry on this.