it is 100% not an ammonia issue nor nitrite. we can scan the forums and find several angry corals posts in years old tanks, angry corals happen.
hunt elsewhere, its not these two variables. if that was my reef, it would have all new water today matching temp and salinity and there'd be no headache. I realize others will not do that, sixteen other ways will be chosen instead. also at no time would I add unprepped fish from a pet store, because them catching disease and not eating rises to about 80% likely and that confounds other troubleshooting issues for the corals.
so in summary I'd never input untreated fish, would do a full water change matching temp and salinity, never test for nitrite or ammonia again on this tank, never dose prime again or bottle bac to it. I'd do fully opposite of all current moves on this tank. I know everyone gives different advice, it makes it hard to choose / aware.
the water column is being chemically souped with additives and bacteria floating in suspension with no where to attach, the bioloading of your water is increasing, totally opposite of what Id do which is instate totally clean water and throw out those two test kits you have, ammonia and nitrite, they'll mislead you into harmful actions.