My last tank everything started dying on me and I never figured it out. Basic params were all great and water changes seemed to make it worse. So maybe there is something to the salt thing. I think I was using reef crystals back then but can't really remember. I know Kent Marine had a known bad batch of salt about 10 years ago that caused a lot of tanks to crash. It had super low alk. Not that that would kill your tank right away but I think a few people panicked and did even more water changes .. thus exacerbating the issue. Anyway with my old tank I always suspected either some sort of chemical from softies, some nasty gas from not cleaning sand/rocks enough or the salt had an imbalance of something bad who knows what. Also I manually dosed and it could have been alk swings with that. Big believer in dosing pumps/reactor now. Although it can be done by hand it has to be done with care. If not already I would stop the Reef energy until things stabilize. Also I assume since things are dying you stopped dosing alk for now. I would take everything down to a basic Berlin system and see what happens.