I believe the main reason to use garlic is that it encourages stressed out fish to eat. In that way, yes it does prevent something--death. However, I don't believe that it effectivly prevents disease in healthy fish. In order to get that effect a varied diet is the only way to go (i.e. different flake, different frozen food, dried sea weed, etc, etc). I'm also a big believer in fatty acids, like Selcon. I think it does wonders for fish as well as invertabrates.