I imagine he's referring to using a single QT to work for fish and corals. That's what I'd have to do if I ever wanted to QT fish. You can treat fish in a QT with copper but want to make sure that anything you have in the tanks is removed before you put coral in there. You can rinse a product like Matrix but there's still the potential to have it leach copper back out. Of course is the fish are good, there's no need to treat with copper and it won't even be an issue.