The hippo tang grows over 10" long, and does not thrive longterm in a small tank. The tank size is probably the root of problems. They are a far-swimming, high speed species, and keeping this species should wait on a very large tank. A cascade of troubles can come from one fish whose health is declining: things it may have succumbed to then spread to others. Your store should have advised you how much room this species needs as one of the largest fish in the hobby. You did your best to do the right thing, but without the critical advice. Suggest you devote your tank to inverts and corals for a number of weeks, then start some blenny/goby/or assessor types in quarantine. The inverts can't catch the maladies that might be present, whatever they were; and I'd also run some water changes and get the parameters to 5 nitrate or below and 8.3 alkalinity, to be sure fishy skin has ample slime to defend against any residual problem.
