Its the garlic juice or oil that is what we are looking for. If you have a garlic press the key is not crush the garlic with any speed but to slowly squeze the garlic cloves in th epress to extract the oil from teh clove. This oil is very pungent and will leave the room with a great garlic smell
I used 2or3 cloves so as the press was full and then slowly squezed catching the oil in a small saucer. I found that I could press the garlic twice before the fibers where to fine to squese anymore.
Soak your food in this pure galic oil and then feed the food to the fish. The comercial garlic additives like was mentioned above are basicly the same juice for the most part but they might have added other things aswell.
The use of garlic in aquairums is not scientific by any means. As was mentioned it wont remove the ich from the tank but more or lesss remove it from the fish. Since the parasite is a hosting parasite it needs the fish as a host to complete its life cycle.