"A fish? No!"
The thing is, that you don't have any context here. I think that "Ryba? Nie!" is perfectly fine. But you can imagine that the whole phrase is a part of a dialogue, e.g. like that:
A: Chcesz rybę? (Do you want a fish?)
B: Rybę? Nie! (A fish? No!)
If the person A used a verb that needed Accusative, and it does seem probable here, then person's B answer makes more sense if it uses Accusative as well.
I added such an option.