Just to clear up some of the misinformation in this thread. Hyposalinity (sg
of 1.008-1.009) is a very solid, common practice for treating ich. It is not dangerous to most marine fish and is often the only alternative for treatment in other types that are sensitive to copper.
In this case, the tang may have succumbed to internal parasites or a number of other infections. What happens is, after the initial infection of ich is cured, a secondary infection by another bacteria is always a dangerous possibility. That is why the water in QT has to be pristine. I'm talking 0 ammonia, 0 nitrite, 0 nitrate, etc. If any of those levels are not 0, water change.
Sorry to hear of your loss Bix, especially the tank. That story is the stuff of many nightmares.