That sucks! I lost one ray to death curl and the feeling of helplessness sucks. Fix it up and save your other fish as best you can, good luck!
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ffs.
pH drops because you don't have enough Kh buffering the water and the biofilter uses up Kh over time. The biofilter goes dormant at low pH, ammonia increases, nitrite increases. At low pH the nitrite will be most damaging and then as you raise the pH back up the ammonia will become dangerous. Fishkeeping 101.
Not seeing ammonia and nitrite readings posted in this thread.. Low pH on its own doesn't kill rays, it's the knock-on effects.