I'd QT all the fish, I really don't see a way around that. I had to do that with my old queen and emperor angels. Left the tank fishless for 8 weeks and treated the angels with hypo. Never had a problem after that. Now I have a different setup with a 6" emperor and he got pretty sick, I have him in QT and the tank fishless again. I tried getting around QT, there is no such way. You can feed all you want, have perfect waters, if ich is in your tank, your fish will get it at some point in time. It's very frustrating, believe me, I know all too well, but it's what has to be done.