Garlic seems to work best as an appetite enhancer - I don't think it's any more than that. There are basically two ways to handle ich - "Ich Management", which means you keep your water quality high, your nutrition spot-on and your stressors low, and "Ich Eradication", which means you pull all the fish into a quarantine tank, wipe out the ich using copper, Chloroquine Phosphate or Tank Transfer Method, and you keep the fish out of your display for 76 days, allowing all of the ich theronts to hatch and starve to death with nothing to feed on - and you quarantine any new incoming fish. Inverts and corals should be QT'ed as well, kept in a fish-free tank for 76 days, to make sure that ich doesn't get back into your display.
In an ich management tank, if your heater goes down, or your home AC unit fails on a hot day, ich may be able to gain a foothold, due to the fish's stress levels. In an ich free tank ... it can't.
~Bruce