Sounds like the tang brought the ich in. We had the exact same thing with our blue tang, we took it back but it was too late for 2 of our fish, didn't have a QT set up at the time and I had to do something to try and save our remaining clown and Koran angel, we got a UV
steriliser and man it totally worked! It's the only thing I can suggest other then getting a QT set up ASAP and cycle it quickly!!