You have a couple of options, neither of which are easy but workable.
It's not practical to quarantine all of these fish together in one tank, unless it would be in the 100 gallon range. You can either purchase several large rubbermaid bins to house your rock, corals and inverts, and perform hyposalinity in your main tank, or you could use a couple of these bins to house/treat the fish, leaving the DT intact with rock/corals. If you opt for plan B, you need to remove the fish anyway. Drain about half the water out into containers. Remove the rock.corals from about a third of the tank leaving a large open spot. Get a helper. I fashioned a tank divider from a piece of eggcrate. Have your helper use a net to gently herd one fish at a time into the open area. Use the divider to trap the fish into this area. Once he can't get back to the rocks to hide, use the net and a capture container to gently catch the fish, and remove it to the quarantine tank. Do this for each fish. It's not hard, just time consuming. The drawback to bins as quarantine is that it's difficult to observe for symptoms from the top down. Each container, whether it housed rock or fish will need a heater and powerhead. You'll also need to have ample lighting for the corals, which is why I would suggest leaving these in the main tank, and treating the fish in separate bins. How big is your QT btw?
Once all fish are removed, replace your rock and corals to the DT.
I'd also ask, on the tang with ich, how many spots are you seeing? Have any fish been added recently, and when was the last fish added? What species of tang has the ich? A possible third approach, and while not popular, is to take a wait and see approach. I would only suggest this if the fish with visible symptoms has a very light infestation (only a few spots). Sometimes, otherwise healthy fish can stave off a parasitic outbreak, so a few spots does not always trigger a full blown outbreak. I've seen it work with success but if your fish is heavily infested, treatment is advisable.