"It is a red fish."
Translation:Det är en röd fisk.
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? :)