Is in this case an article really necessary in the English translation? Can't it also be like an undefined amount of fish that is orange?
Fish is countable in Polish: jedna ryba, dwie ryby, pięć ryb (1 fish, 2 fish, 5 fish). That's why there is 'an' in the sentence.
Just to rephrase it, to make stronger point for this particular example:
In Polish sentence To jest pomarańczowa ryba, the pomarańczowa ryba is just one singular orange fish. And because of that the meaning of the sentence does not allow any amount of fish and needs an article.
If there was more than one fish, the sentence would be "To są pomarańczowe ryby".
Just out of curiosity, then, if you had some fish on your plate (not one fish or several fish, but just a piece of fish), how would you say "this is orange fish"?
You would say just as it was one fish (see the sentence at the top).
If for some reason,however, you really wanted to specify that it's just a piece, you could say "To jest porcja pomarańczowej ryby".
Kawałek is also good, although „porcja” has a stronger implication that this piece is meant to be eaten by one person.
Why isn't this "To jest pomarańczową rybą "? Doesn't the verb być take complements in the instrumental case?
You use instrumental case after „jest” when the sentence has a defined subject. If the subject is not defined beforehand, you use nominative.
„To jest pomarańczową rybą” – It is an orange fish
„To jest pomarańczowa ryba” – This is an orange fish
Or to say even simplier: sentences that translates as "this is" use „to jest”, and you almost always use nominative after „to jest” or comparative „to”.
For Czech speaker, this sentence in polish sounds like if the fish was made of oranges :D
No. That sentence is quite different grammatically. It changes the subject from "This/That/It" to "This fish".