As said, no fish in the dt for at least six weeks should make it a parasite free tank, however, if the ick is hanging on in qt somehow and not completely wiped out, which in my opinion could be some parts the treatment and some parts the fishs ability to fight off the parasite before it completes its cycle, you will just carry it back on the fish when you put them back in the dt. But sounds like you give them a good diet, I wonder if your temp fluxuates a lot in qt which could cause more stress in the fish. Even so, I understand that the copper should have done the trick and so should hypo. I understand your frustration.