What kind of ray did you buy?
I had posted this in another thread the other day:
Hey guys, Rays are illegal in CA. Teacups are just a description of a small ray.
Generally, what the LFS call teacups are the common rays out of peru and columbia. Generally, a white/grey back ground with a very cool design. Teacups are the cheapest you will find, but also the hardest to keep.
Motoro's tend to be the heartiest of rays but do get large. If the disc gets to be 18 inches you will definitely need a larger tank. Its the width that matters with these guys.
Rays will dig up your plants. Put plants in the back corners surronded by drift wood and the rays will have a harder time digging them up.
Finally, keep in mind that rays are predators and you can keep them with discus, I do. But, keep in mind the size. I found a gorgeous cobalt in my last shipment and put it in my show tank at 2.5 inches. I got up the next morning and found that its face and half of its body had been eaten. 2.5inch discus with a 11 inch stingray didn't work out =( If you have a large ray you should be buying larger discus that are quicker and larger than the rays mouth. Typically, a ray won't go after something if it knows that it can't fit in its mouth.