Formula One makes a sinking pellet that has garlic. While I don't use pellet too much because of the phosphate issue, I do feed it if I have a new addition to the tank.
You should be quarantining your new fish to be sure they don't have ich. But just as a prophylactic, it's a good thing. Garlic contains some volatiles that, at least in humans, can exude through the skin: there is a case of pottery workers in central France who were found to do so because of a genetic quirk [their fingers were staining the pottery black]. So it's uncertain whether this happens in fish, but if it does, it could suggest a mechanism by which it could discourage the ich parasite.