If you have already put copper in your display tank then you might as well just do your treatment in that tank. Once you add copper you really can't ever do corals or at least I don't recommend it. And I wouldn't ever use that live rock for a reef either. After your treatment do like 40% water changes for a good couple of weeks and then test your copper levels to make sure there back down. Now that you have the qt when you buy a new fish have it stay in the qt for a couple weeks. And try to keep the fish stress free. Then you should have no more ich.