If you have a fish-only tank, then you can do hyposalinity directly in the display. If you have just a few corals and inverts, then I would just remove those and go hypo on the display. If you have a fully stocked reef, then yes, remove all of the fish and go for copper in the QT. My preferred treatment is Cupramine by Seachem as it's easier on the fish, and the concentration stays very stable, even with skimming.