Garlic is just to help a not-eating fish eat(the flavor), and it's supposed to boost immune system. It may help keep up resistance to ick, but as a cure for a current infection, probably not.
I've started off a few hard-to-eat butterflies & tangs by crushing a small clove and then I rubbed a frozen cube of that algae supplemented hikari brine in it for a sec, then defrosted it in a cup of my tank water and fed. I'm not certain on a fish OD'ing on garlic, so i do it lightly, just enough for flavoring.