Ya you can keep a lot of rays in there for a while. Not for life, mind you, but for a while yes. Get a small ray (at most 5" if possible I know some contacts that may be able to get you one, PM if you like). Rays have the same growth rate of max. 2" per year. So either a yellow, cortez, or round(cali) ray would do. If your tank is at least 75 degrees(which I'm guessing it is becuase most tropical tank's temp) then you can't do the round(california ray). Blue spotted get a little to big, IMO. Send me a PM for more info.