Sadly, there are parasites with long life cycles and Cryptocaryon and oodinium have an approximate 60 day cycle so QT for 3 weeks may have not been long enough to prevent this occurrence. Copper is only effective for one of the stages of the parasite's reproductive stages.
That all said, and assuming that this is a large aquarium because Tangs get very stressed in smaller aquariums, something has stressed the fish that they succumbed to a parasitic infection. To determine "WHAT was the stress" better, please give us current water parameters including temp, stock, tank size, etc.
Be prepared to remove the fish to a sterile hospital tank to treat as the best med for parasites is copper or copper based and will get absorbed by your coral decorations and kill your inverts.