I have further updated the popular "Aquarium/Pond Fish Nutrition" article
Here is a quote from the article about Garlic:
I do not have good evidence of the benefit of Garlic for parasite prevention/treatment, however I do have good scientific evidence of the use of garlic for bacterial treatment (which may at least allow for a stronger immune system to aid in treatment of parasites).
Garlic contains the active (& proven) ingredient; Allicin, (a.k.a. Diallyl thiosulfinate)). Allicin is a broad-spectrum agent against both Gram Positive and Gram Negative bacteria, with more gram positive activity (which unfortunately most aquatic diseases are gram negative).
Fish Tuberculosis (Mycobacterium marinum) has been demonstrated to be effectively treated with Allicin. Some human studies show the effectiveness of Garlic (Allicin) in treatment Streptoccocus (which can affect fish).
Reference:
Carl