Read the link provided. Learn the history of how garlic was first introduced to hobbyists.
Someone told me I should repost this,
NO, I don't recommend garlic for treatment of parasites or anything else aquarium related for that matter. The active ingredient in garlic responsible for all it's miracle interactions is the chemical Allicin. Allicin is produced by an enzymatic reaction when raw garlic is either crushed or somehow "injured". The enzyme, alliinase, stored in a separate compartment in garlic, combines with a compound called alliin in raw garlic and produces allicin. Because allicin is so unstable, once it is generated it readily changes into other compounds. Thus cooking, aging, crushing and otherwise processing garlic causes allicin to be decomposed into other compounds. According to scientific studies of garlic preparations, allicin decreased to undetectable amounts within one to six days. IF you're a firm believer in the magic of garlic, you really need to use freshly pressed cloves to get this chemical in an active state. NO AQUARIUM PRODUCT SITTING ON A SHELF IS GOING TO GIVE YOU THE ALLEGED BENEFITS OF GARLIC. The hobby has been bamboozled by marketing hype. Educate yourself and don't fall for the scam.