Rachel,
I have never heard of such devastation. I wouldn't worry too much if the KH is 12.5. That's just a little over the recommended high value of 11.5.
Have you been running any carbon since this began? If so, how often have you been changing it? If this was my tank, I'd change it daily and be doing at least 30% water changes once a day, maybe twice if possible, with RO, RO/DI or distilled water along with your salt mix. The new salt water should be aerated and heated to tank temps for as long as possible before going into the tank.
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