That's what people always advise, garlic is like the vitamin C of the fish, so it should do something to the then.
Gives them bad breath
Garlic won't cure ich, only thing to cure ich is treat livestock and tank, if you wan't to get rid of ich, you have to QT your fish and treat with HYPO, cooper or tank transfer method and leave your main display without fish for at least 10 weeks to be sure all ich is gone.
But, the thing is this can be done if you have few/small fish, otherwise you probably don't have were to put all your fish comfortably, and here is were the hypothesis of my theory begins, people with big tank don't QT after they get ich because they simply can't, and after a while nothing happens.
I Have a 180 if I got Ich I would pull the fish.
If you keep good husbandry and your parameters are in check, your fish are healthy and fat, you probably won't have any problems with ich, i don't know anybody with this conditions that have lost any fish to ich, and i have a lot of friends with SW tanks here, not online guys, real long term tanks.
I have 2 friends that have 15+ and 12+ years and the hobby, those are the most experienced reef keepers i know, have always told me, don't worry about ich, just check your parameters and keep your fish well feed and everything will do the rest by it's own.