"Fish" is singular and plural, so "the fish" and "these fish" are both correct. Note that "the fish" could also be a plural reference, as in: "All the fish on this table are fresh." ("are" meaning "they are") A little more information from the website grammarist.com ... Fish vs. fishes The plural of "fish" is usually "fish", but "fishes" has a few uses. In biology, for instance, "fishes" is used to refer to multiple species of fish. The word fishes' also serves as the plural possessive for "fish" -- e.g., "Hundreds of fishes’ eyes stared at us." And of course, fishes is the present-tense verb (e.g., "He fishes in the river"). "Fishes" also appears in the gangster movie idiom "sleeps with the fishes", used to indicate that someone has been [killed].

One fish: Is the fish cute / Is this/that fish cute? Plural fish: Are these/those/the fish cute? If you were asking if a kind of fish was cute, you would say Are <type> cute? Example: Are clown fish cute? Are goldfish cute?

Both are correct grammar. The first is singular (only one fish), the second is plural (more than one fish). The noun fish usually has no plural form other than as explained by Grok.