Myth. In fact, studies have shown that garlic can cause serious issues for fish. IIRC, it affects the liver (but don't quote me on this)
Edit: taken from one of my posts a couple of days ago...
First of all, you need to confirm that it is ich. If it is, then you have two choices:
1 - leave everything in the tank, feed well, cross your fingers, and hope that all of your fish survive.
2 - remove all of the fish from your tank, follow the ich treatment protocols listed in the stickies, and leave your tank fallow (fishless) for a MINIMUM of 72 days (some say 76 days)
Over the counter ich remedies like API super ich do not work, nor does garlic, ginger, or eye of newt.