Derm,
I used garlic over the winter to help defeat an ick outbreak. I added a few drops of Kyolic Liquid aged garlic extract with daily feedings to control and ultimately beat it in a hippo tang and
perc clowns. It took 10-12 weeks, but I'd credit it with the win. Note that I was also working very hard to improve every other water quality parameter at the time, but I think the garlic was key.
Be careful just picking up any-old garlic capsules or extracts a lot of them have oils in them that will result in a sheen on the surface. That can greatly reduce oxygen transfer to your water in a big hurry.
The Kyolic Liquid I found at GNC and other health food stores. The other one that I found to work well with the food was the juice from a jar of minced garlic that you can get in the produce section at the grocery store.
Daryl