What else do you have in your display tank? If you have inverts and live rock, could you remove them and put them in your QT? If so, then you could do a hypo on your display tank and kill ALL of the ick.
If you have live sand, it would be good to remove that as well. It might be the best option for you?????
The Ick will not just die out by itself unless there are no fish in the tank. Even if you don’t see them, they can be there. Plus, all it takes is one little ick to be attached to the gills of one of your fish and you’ll never know it’s there. But the minute you put that tang back in, he’ll be covered in it.