How big is the tank?
How long has it been up and running with fish?
What other fish are in there?
Describe your tank...details please. Type of rock, substrate, filter, etc.
What are the tests reading and what for...numbers please. 'Fine' for some fish may not be 'fine' for others...especially the shark family. Very sensitive to water chemistry changes.
It is possible that if all the requirements for that sting ray is met, it may just be in an adjusting period. Acclimation can take up to two weeks. You should set up a quarantine tank for new fish.
Watch how the other fish are with the ray. Is anyone picking on it?
Keep in mind rays need lots of room and get fairly large. If their requirements are not met, they can quickly become ill and die.
, rays do not belong in aquariums. They are fantastic creatures; some even majestic like the Manta Rays. They capture our attention mainly because of their unique shape. However, unless they are displayed in Sea World sized tanks for education, they serve no purpose being in an aquarium. They get far too big and most die premature from captive related stress. Hope it does ok. Good luck.
We, as a people, know so much more about outer space than we do about our own oceans. This lack of knowledge can very well spell the dangers that lay in wait for us.
The oceans surely would swallow us before a rock comes down to smite the planet of it's life.