stringrays are frequently very difficult, just so you know. They are notoriously poor eaters. Furthermore, they need a very specialized tank. Very wide and long, but not necessarily tall, and very little structural decoration, with a very fine sandbed. Most people I have heard of try to squeeze them into a reef tank with tons of live rock, and the animal just fades away and dies due to stress and innappropriate housing. If you going to do this, get a tank that's a good 3 or 4 foot square (at the minimum), and only put a few small rock bommies, if you must. Like I said, if you want to go this route, you'll have to specialize.