Was this bought in Livonia by any chance ? Tangs are known to carry velvet gill disease. It is almost impossible to see and is often verified by a tissue sample. I see no problem with a small qt. either. It's good you did QT the livestock. I only asked about the location because my wife purchased a angel for me for Christmas and did not QT it. She slipped it in the tank to surprise me. Boy did we get a surprise ! We have now lost 11 fish ! QT is a must ! We are in the processes of fighting this now and do not even want to think of the money lost ! One of the fish we lost my wife has had over a decade ! Sorry for your loss but be grateful it passed away in a controlled area instead of your DT.