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.
NoTrueScotsman 
Not only is “fish” both singular and plural, but it is also verb, both transitive and intransitive (“I fish tuna” and “I fish for a living”). English also has a grammatical construction where you can insert a noun/verb phrase within another noun/verb phrase. For example, if the boy is walking a dog that barks, you can say, “The dog [that] the boy walked barked.” And you can do endless iterations of this. “The dog the boy the girl kicked walked barked” is grammatically correct, albeit awkward.
As a result, any number of repetitions of the word “Fish” is actually a grammatically correct sentence.
Fish! (the imperative)
Fish fish. (i.e., fish catch other fish in order to eat)
Fish fish fish. (i.e. the thing that fish fish is fish).
Fish fish fish fish (i.e. Fish that fish fish also fish.)
Fish fish fish fish fish (i.e. Fish that fish fish also fish for fish).
Etc.
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