I think that any call of "My Ray (or any other fish) is sick" should come with bang up to date readings for Ammonia, Nitrite, Nitrate, PH and Temperature.
I have found that most fish illnesses or deaths can be largely attributed to the water - parasites often only begin to "win" once the fish is ill from poor water.
I would test Nitrite and Nitrate in this case and whilst many say Nitrate is safe...it is not. Prolonged exposure to anything over 25ppm to 50ppm will lead to some form of health issues. Some tap water comes out the tap at close to 50ppm therefore if the Rays are producing waste it is easy to get up to 100 to 200ppm in a matter of 2-3weeks.
I have lost expensive fish to Nitrate and this is backed up by scientific research but it is taking a while for the hobby to listen as too many books still say that Nitrate is harmless. (there is a reason that safe limits for humans are set by water authorities and we do not live in and breath the water or have to osmoregulate)