First of, Welcome to Reef Frontiers!!!!
There are a lot of supposed "Ich Remedies" on the market that advertise themselves as "reef safe." However, IMO, if it'll kill Ich, it'll kill other beneficial reef inverts, zoaplankton...etc. Then there's the Ich remedies that will kill everything in your tank, except your fish. Unfortunately, the only safe way to treat Ich and be 100% certain that it's OUT of your display tank, is to remove ALL fish from your display and treat them in a QT tank, with either hyposalinity or copper treatment. The problem you'll run into now, is that if the Tang has Ich, there's now Ich in your system. Even if you just removed the Tang and treated it, you'd still have ich in your system. That's where the suggestion of treating all fish come in. You'll need to leave your display "fallow," NO FISH, for a period of at least 8 weeks. This will insure that any Ich in the display dies, after going through it's reproductive life and not having any fish to latch onto. There's TONS of very valuable information available on Lee's Marine Fish forum. I'd definitely recommend you read his "stickies."
http://www.reeffrontiers.com/forums/forumdisplay.php?f=15