Water changes are way too infrequent the way you are doing them at 25% per month. I do 50% changes every 1 week on my ray setup. The diet is insufficient as well.
I would run out and get some test kits and see what your water parameters are like before going through any procedures. Helpful information would be what is your, pH, ammonia, nitrite, nitrates. It's tough to diagnose the problem without water parameters. Without the readings, you could end up doing more harm than good.
Also, what filtration do you have on that tank? If your filtration is inadequate, there may be ammonia present in your system which in severe cases would cause ammonia burn. If the spots are appearing out of nowhere, this could be the case. How are the disc edges of the ray?
I would run out and get some test kits and see what your water parameters are like before going through any procedures. Helpful information would be what is your, pH, ammonia, nitrite, nitrates. It's tough to diagnose the problem without water parameters. Without the readings, you could end up doing more harm than good.
Also, what filtration do you have on that tank? If your filtration is inadequate, there may be ammonia present in your system which in severe cases would cause ammonia burn. If the spots are appearing out of nowhere, this could be the case. How are the disc edges of the ray?