Feeding saltwater fish Garlic and Ginger is total nonsense, if not even harmful.
Garlic is beneficial for humans, but already harmful for cats and dogs. So the assumption it may be beneficial to saltwater fish is rather far fetched (or outright foolish). There are actually reports that indicate that feeding saltwater fish garlic causes them liver damage.
As for treating ich or not - that depends on which kind of fish you plan to keep and if the infection is escalating or decreasing.
In general, if you plan on keeping tangs, especially more than one, or other easily stressed fish, you should rid your system of ich completely or you may have constant problems.
If you have fish that are less prone to stress and do not plan to add anything and the infection is declining you could just leave it alone. With some luck the ich may even die out.
It worked for me several time before and I never had losses. But then, I'm extremely mindful of the fish I put together and see that their needs are satisfied and they are happy. And I avoid tangs like the plague.
When in doubt it's better to eradicate it or even better, not let it into the tank in the first place.