Sorry for the late reply...I did not this anywhere but if someone asked already, I am sorry... How long have you had the ray?
If it is a new ray, there may be nothing you can do, sometimes shipping damages a ray (or any fish) in ways that we can't visually tell untill the health starts to go...
If the ray is swimming in the top non-stop, not settling down at all, I believe that it will not survive, sorry, just my personal experience. I also don't believe that this would be your fault, or lack of research, nor do I think that this is the sellers fault, who knows what happens to boxes while en route to your home?
If you have had the ray for a while and suddenly it is getting sick there may be hundreds of causes, I agree with holywood (not his attitude) that we need more water parameters to be able to me more helpful. I don't agree with the temperature issue, my rays are at 86F and very happy, same as all my discus.
Water change is a good idea, salt is also excellent, good luck, hope it pulls through.
Rod