The damage is done my friend. I went through a similar situation. You could quarentine the tang, but regardless if your other fish show it, you'll still have ich in the tank.
If you remove the infected tang, but then add it back to the DT when healed, or add another fish to the DT for that matter, they could come down with it too. I learned that the hard way after losing a Kole Tang, a White Cheek Tang and 2 different wrasses.
The only sure-fire way to fix the situation, is to pull all the fish out of your DT and leave it empty for 6-8 weeks. Be patient, and quarentine your fish properly before adding them to the DT.