If I have to guess the cause of the death for the fish, it's probably the drop of pH that did it.
Yes, the SG is high, but if it's a gradual raise due to the evaporation, that shouldn't kill the tang all of a sudden.
Yes, you shouldn't have phosphate, but that does nothing to the fish.
Yes, you shouldn't have traceable nitrite, but like stated, nitrite aren't as fatal as it is in FW.
Yes, your nitrate is very high in the tank for SW, but there are several studies out there saying fish can tolerate nitrate level even higher than that, it's the inverts that are far more sensitive to nitrate.
So considering all these facts, I would slowly add the pH increaser as the fluctuation is far worse for critters to fare.