If your other fish are scratching, you have to treat them all or he will get Ich once he goes in the display. So you can either treat the whole tank with copper or move all of your fish to a hospital tank.
I used copper to treat all of my fish and then I left my display fallow (no fish, just corals and inverts) 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).<br />
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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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