"It is a red fish."
Translation:Det är en röd fisk.
Det is the formal subject here, much like it in It is raining. So it doesn't refer to the fish, and it doesn't matter whether the thing is neuter, common gender, or plural.
The way I see it is you would use only "den" if it was acting as a definite article for the En noun "fisk-". (And "det" for neuter gen. nouns)
In this question, it's asking for an indefinite article "en" for fisk and "Det är" just means "It is".
However, I think we would use "Den" if the sentence was "The red fish" = "Den röda fisken".
Any advanced Swedish speakers wanna pitch in on this? :)