The only thing that I have read that makes sense to me is that it only entices the fish to eat the food. Nowhere I have found any studies documenting the fact that garlic has any impact whatsoever on their health. It just makes the food more appetitizing, hence they eat more and the food makes them get better, not necessarily the garlic itself.
There has been no downside that I have read about, so if a fish isn't eating, or isn't eating much, try adding garlic to spice it up for the fish. But don't expect it to cure anything though.