Sorry to hear. Im no expert either but I'm going through a similar issue. Have lost quite a few sps but lps/softies show no signs of being affected. I stopped testing my parameters often a few months ago and my sps started dying. Either from the base or in the middle... but never from the tips.
I suspect an alk swing with 0 nitrates/ phosphates was the culprit. My digis, pocilloporas, and a few others were however were unaffected... but I lost about half of my sps. My tds meter crept up to 1ppm after the last stage recently... so maybe my ro water is an issue as well.
Hopefully it was the lack of nutrients and alk swing... cuz I'm dosing nitrates/ phosphate now and seems like an easy fix. Setup auto dosing, awc and am testing everything daily again.
Hopefully u get it all worked out!