I can never make up my mind. I've got a backdrop which is a kind of 'rock wall'. It's effectively lots of very dark greys, kind of slate coloured I suppose.
Anyway, they say (and I think it's correct) that a plain(ish) backdrop draws your attention to the fish, rather than the backdrop, and a dark colour enhances the colours of the fish, because there's a stark contrast.
As kaz petts says, the backdrop does need to be attached well, because otherwise you can get gaps between them, and that causes a weird effect, often mistaken for water trapped back there.
Of course though, any backdrop, or decor of any sort is a matter of opinion, so there's not right or wrong answer.