What ecological niche do disk shaped fish exploit?
I believe what you're referring to are fish with the compressiform body shape (fish with a narrow flattened body). The advantage to this body shape is that it allows the fish to swim very quickly for short periods of time. This would, I assume, be an advantage both as predator and prey in that it can use a quick burst to escape from something else trying to eat it and also allow it to use a quick burst to catch food for itself.

--added--

I just re-read your question and caught the "ecological niche" part, which is different from evolutionary advantage. I'm not 100% sure regarding this (I'd say 80% sure though), but I don't think that an animal has to fill a specific ecological niche in order to survive - it just has to...survive. Surely animals that fill specific niches can gain an evolutionary advantage by filling a niche, but it is not (to my knowledge) a requirement.