I use Garlic Extreme by Kent and like it. The fish really like it. I have read some people finely chop fresh garlic. I used the oil a few years ago and it made my tank oily. This was just my experience.
also do a search on Terry B info on beta glucan. He prefers this to garlic. We use Kents garlic, but since its so pricey when we are done with the bottle we will switch to beta glucan.