Garlic and other suppliments have been used to "boost" immunity for some time. A lady here swears by Beta Glucan. Having healthy fish is the best way to avoid parasites.
I don't this it is a cure all though. I don't think you will be able to keep the ich from feednig on the fish by feeding them garlic only. Removing the fish and feeding them garlic enriched food will help them fight off any ailments they have and boost thier immunity later on.
*180 gal Display, 100 gal basement sump, 33 gal refugium, 3x250 MH, 2x160 VHO actinics, zoos, some softies, LPS & lots of acros and other SPS.
*100 gal prop tank plumbed into main system w/ 2x96 PC lights and 1x150 MH,