Well if you get more fish and just put them in the qT tank. They might still be carrying Ick or other parasites without showing any signs. If your treat, your sure your not adding any sick fish in your display tank again. You can just observe for a while and then put them in the display if they seem ok. But you'd be taking a chance. Up to you.
You could also use hyposalinity instead of cooper. That's what i like to use since it's easier on the fish. But you need a well calibrated refractometer and make sure the salinity is stable.
Again, please read the stickies in this forum. They will answer all your questions and help you understand what's involves in treating fish for different deseases and parasites.