Saltwater Aquarium Fish for Marine Aquariums: Fiddler Stingray
Read the above link to get a better idea on what is needed on how to keep rays in the tank. I understand that this is just ONE type of ray, but aside from their individual sizes and gallon requirements, their living and eating principals are generally the same.
Also, alot of these ray sites say Expert Only, for a reason. These are not by any means the easiest fish to take care of. I would do ALOT of research on rays and their living habits, eating habits, how they like the water, etc.. before you even consider taking care of one. I would also suggest going bigger than what the minimal gallon suggestions are because in the rays case, the bigger tank, the better.
Hope this helps.