Does garlic kill ich, or keep the ich from re-infecting the fish? Most likely not. Does it help the fish fight the ich off, probably, but then again keeping a fish eating and active goes a long way in helping a fish fight off ich. You'll also find that most of the food sold nowadays is load with garlic already, so you probably don't need to add any more. Stress and water conditions do play a part in helping the fish fight off ich, but, if you don't have ich in the tank, then it doesn't matter the level of stress or what the water conditions are, the fish isn't going to get ich. You see this all the time, horrible conditions, fish is small tank completely stressed out and no ich.
The tanks that seem to have ich, where the fish are living with it, usually have things that can help keep it in check somewhat. Things like fast water movement, corals and inverts that can trap and eat the spores, and filter
socks, helps keep the ich in check.
There are also variations of ich that are pretty deadly and kill very fast, and usually when introduce, by adding another fish, take down the existing fish in the tank, regardless of how healthy the fish is. Plenty of case of wipe out in tanks were the fish are super healthy, great water conditions and eating. While other variations seem to be less fatal, and fish can live with them, even to the point where some become immune to the ich.
The real question is what we're seeing on fish is even ich in the first place. There is a good chance it maybe something else, which just happens to show similar symptoms. Hippo Tangs will always show signs of ich, but no other fish in the tank will get it. So it's possible that it mayn't be ich or could just be a strain that attack just Hippo Tangs.