I have had my yellow and my powder brown tangs get ich recently. The advice I've been given was to soak their food (both eat seaweed, one eats mysis, the other brine shrimp) in garlic and vitamins. I use Garlic Power, which is a garlic extract, and Selcon, which is a multivitamin solution. I've had great success with my Yellow (no signs of returning ich for about a week), but my Powder is still fighting it. I would try this first as I've been told that once they fight it and win, their immune systems are more apt to successfully fight it again.
Harsher treatments include Hyposalinity and Copper. Both (i've been told) are very stressful to the fish and need to be accomplished outside the main tank.
