I think there is an issue in regards to garlic that needs to be cleared up.
Garlic may possibly help build up a fishes immune system, and it may not.
It does not cure Ich. A recession of Ich while using garlic is most likely purely coincidental. Garlic is intended to be used to stimulate a fishes appetite and the labels on the commercial products say so. In combination with proper diet, which it (garlic) enhances, and the use of vitamin suppliments the fish stand a much better chance of surviving a bout of Ich if it is being treated correctly.
