I don't think you have to set up a quarantine tank. These tangs are tough and you would have to quarantine all the inhabits for up to a month or more. Keep the tangs well feed in clean stable water temperature at about 78 degrees. Start now by giving them some sea veg.
I agree that at this point QT may not be the best option. Especially if you dont have a fish trap to make capture of the fish quick and easy.
If you do plan on setting getting additional fish in the future, a QT may be a good idea to set up now. I think its more important to keep a new fish to get it to eat, and get it back up to full health before adding to the rest of your tank. At this point you have ich in your DT as most people have, its just a matter of keeping the fish healthy and the immune systems working optimally to prevent an outbreak and possible loss of livestock.
If you have ich in your system I have found that the best solution is to put a few drops of Garlic Xtreme by Kent in the food. The garlic will entice them to eat and strengthen their immune systems thereby allowing them to beat the ich. In my experience this has worked 100% of the time. I never setup QT tanks and had the same 3 tangs for about 8 years. You could additionally get garlic infused algae sheets for them as well. Cleaner shrimp will also help.