There is a garlic extract product that you can buy made for the hobby to soak fish food. However, you can make your own by soaking food in actual fresh garlic that you crush or, as stated above, you can get garlic extract from the grocery store, or garlic gel caps from the health food store to soak the food in. Be sure that you use a product that has only garlic in it, not other additives.
Garlic is used to assist in treating parasites and, to a lesser extent, bacterial problems. It is also thought to be an immune boaster [which is why human beings buy it as a supplement].
It is not necessary to feed your fish garlic soaked food on a regular basis. Perhaps when you first buy fish and they are in a QT setup, it would be a good practice to feed with garlic, and, of course, when a fish is suffering from ick or other disease processes.
Garlic is a new additive used in the hobby so the long term effects of using it, particularly everyday, is unknown. It is a natural supplement. It is used on dogs to treat parasities as well, specifically flees, so it obviously has a proven effect on parasite prevention. For humans it is considered to be an immune boaster and is commonly sold in health food stores as a supplement.
I would say that you would be betting a good bet if you use it, but you may want to use it only as necessary, as I describe above.