I had several bad experience with medication on rays. for some apprend reason everytime I try, my rays ends up dying. Rays are the most difficult to cure. They are not as tolerant as other fish. Once they are sick, the only few things you could do are to increase the temperature, add some salt, and most importantly prey.
To be honest, it is only a matter of time before the disc starts to curl and the fish die. I just hope this does not deter you from keeping rays. We all had bad experiences, mostly with rays. However you will gain one good thing from this unforgetable incident and that is experience.
Most rays are shipped in poor condition. You have to examine them before you make your selection. Ask question.
Some tips for next time:
1. check to see if the ray is active, are they moving around.
2. check the thickness of the tail, this indicate the fish is eating alot and are healthy.
3. look for any physical damages, small rays have difficulty recovering from injuries. They tend to stress more.
4. check for any parasites, remember rays can't tolerate much of medication.
5. check for disc curls, once it curls there is no coming back.
6. check between the eyes. look for indentation, this is a sign of poor condition.
7. if given a chose between tank breed and wild caught, chose tank breed. They are more tolerable to water condition, such as higher ph.
Good luck and I am sorry you had experience this terrible situation.
VESS