If you search the ray forum threads you will find some people suggest Melafix is ok for use with rays, while others say it is of limited benefit and can cause irrritation to rays.
Most people do recommend using salt with rays but I'm not sure whether discus like it!
IMO, daily large water changes is the best option as it's quite likely that inadequate water quality is at the root of the problem. Failing that, if you feel meds are necessary it would be easier all round if you could quarantine the affected fish in a smaller seperate tank for treatment. If you're concerned there's an underlying infection then a broad spectrum antibiotic may be of benefit.
Good luck
Most people do recommend using salt with rays but I'm not sure whether discus like it!
IMO, daily large water changes is the best option as it's quite likely that inadequate water quality is at the root of the problem. Failing that, if you feel meds are necessary it would be easier all round if you could quarantine the affected fish in a smaller seperate tank for treatment. If you're concerned there's an underlying infection then a broad spectrum antibiotic may be of benefit.
Good luck