I always think of quarantine as the process of holding a new fish, to watch for signs of illness or parasites, before adding to the main display. If you have a fish with ich, and it is in the main display tank, your best bet is to remove all the fish and place them in QT (or what I call hospital tanks if they already have a disease or other problem). Your inverts and corals can be kept in the main display without treatment, they don't act as a host for ich. If you keep your main display totally fishless for 6 weeks, the ich parasite will not have a host to complete it's life cycle and will die out. In the meantime, you will need to run a treatment to kill the ich on the fish that are placed in QT. A lot to go through, but this is the only way of making sure your fish don't have another outbreak weeks later. Also, make sure you QT any new fish for 4-6 weeks before adding to your display tank. It's easier treating one fish that is not already in the main tank than it is to remove all the fish in your main tank and then have to treat them all.