Give your fish a fresh water dip for five to ten minutes, then try putting him in a water only tank with a copper medication. Remember that you need to get rid of the ick in the main tank before you can put him back in or this will just keep happening.
Also, regal tangs get ick all the time, really their is nothing you can do. Garlic is the best way to speed up the process of the fish dropping the ick, but it never really gets rid of the ick. Try cutting down on the ammount of food you are feeding your fish to once a day and every so offten no food for the day. Only alge for the tang. If you have cleaner shrimp, the lack of food in the system will make them clean the fish double time to feed themselves.
I found by doing nothing but cut back the food and just letting nature take its course was the best method. My tang is doing great, no ick for over a month. Best of luck!!
CP
Vancouver