Some people use commerically made products like Garlic Guard by Seachem, and Garlic Xtream by Kent. Others add fresh garlic to homemade food mixes. I've never saw a need to use garlic at all. But then, I'm "old school".
I used garlic for a while per Wattley's instructions. I took fresh chopped garlic and mixed it with beefheart pretty much by smashing the two together. The whole purpose of this was to decrease parasites and cure black discus disease. The jury is still out on whether or not it works.
garlic stimulates their appetite and is great to prevent any stomach issues. I've been using garlic for years and wouldn't do without. Use fresh garlic only though. I tried the dried stuff and the fish don't really go for it