I used copper to treat all of my fish and then I left my display fallow (no fish) for 13 weeks. Since then I have used the transfer method before adding new fish. No sign of ICH even with my new Achilles Tang (notorious for Ich).
Since I started using the transfer method I have thought that the safest way to treat affected fish would be to transfer them between two fully equipped large containers (30-40 gallon tubs) every 2-3 days, drying the containers, air stones, PVC hiding tubes and heaters in between. It avoids the stress of copper and hypo, and fish can be easily caught in a clear container with little stress (like at your LFS). Each transfer leaves the fish with less spots and no chance of reinfection. Theoretically they should be clean after 7 days, but I'd continue transferring for 12-14 days to be safe. Search and read about the transfer method, it works and it's awesome!
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