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.
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”.