Ich is 100% curable in a tank. There are 5-6 stages in the life cycle of Marine Ich. It takes roughly 5-6 weeks for the parasite to complete it's full life cycle. Meds are only effective during certain stages of the life cycle, However the parasite has to have a host (Fish) to complete it's life and reproduce. It only needs the host in the later stages, therefore it can live on corals, LR, Inverts, sand, etc... for the first3-4 stages. After that it has to find a fish to attach to. Without the fish, it can not complete it's cycle and will die. If you take all fish out of a tank for 2 months there will be no Ich in that tank at the end. If you introduce a new fish with the parasite, you will re infest the tank.
If you treat all new fish for 2 months in a QT tank with copper, you will ensure the copper kills the parasite in all stages. You must maintain the copper at a min. of 2 ppm though. Anything lower will not kill the parasite.