A 6x3x3 is about 400 gallons, would be great for one of the smaller ray species, true hystrix, scobina, or retic. Male motoros or marbles would probably be ok as they don't get much over 12", but females wouldn't fit in there very well. Whatever the "Big Fish" is you will need to make sure its safe for the rays.
It does sound like you probably need to do some research on rays before getting one, would recommend reading through all the stickies in this forum at the least.