As long as he is otherwise healthy, he should be fine...but stresses can often occur. Normally, fish can tolerate periodic outbreaks of ich and clear themselves without intervention. I think more fish are killed be overly-well-meaning keepers that dump the pharmacy at the first sign of trouble. It sounds like you are going with a more conservative route which is usually best. The garlic based foods have a lot discussed both ways. I haven't seen anything definitive done, but it won't hurt and may help. I think it is mainly supposed to be dosed into foods. If the tang begins looking seriously unhappy, it can be moved into a qt tank for about 2 weeks with half-strength seawater and given formalin dips on 3 alternating days. The good news is that fish do gain an acquired humoral immunity to ich, so if it is removed and treated back to full health, it will have developed a measure of protection against further attack, even if the tank itself is not fully cleared of ich. There was a good article on ich a few issues back in reefkeeping magazine.
Good luck with the fish, that sucks, tangs seem to be ich magnets.
