Just sharing my experience with ich.
In my experience, the best way to combat ich is to improve the tank condition. Perhaps instead of looking around for solutions to kill ich, why not take a look at anything in your tank that is not good?
My blue tangs were like ich magnet but after i've improved the tank overall water flow and eliminate all blind spots, it never happened without any help of garlic. However, neverthless, in my experience garlic is good form of prevention.