I have to agree with RBU1. If you have fish with ich then you have the crypt parasite in the tank. If you take out the two infected fish and treat them you'll cure them of the ich. But when you return them to the DT they could wind up getting infected again. The only way to cure your fish of ich is to move all of them to a QT, treat with copper or hyposalinity and allow the DT to sit fallow long enough for all the crypt parasite within the DT to die. I believe at 8 weeks there's a 99.9% chance that all crypt parasite are dead, but there's nothing wrong with waiting 12 weeks. There's no such thing as too conservative. BTW, you should also not return your fish to the DT unless there's been no sign of ich for 4 weeks after the end of treatment.