IME I use garlic in the water and on the food. I have never lost a SW
fish due to ICH and I don't quarantine, unless the fish is getting stressed by the other fish. When I stocked my 180 and spotted the first signs I added so much in the water (a little at a time) I could smell it. Every time I fed the tank, I would soak the food, with a squirt for the tank. After a couple of weeks and a couple of water changes all was right. now if I see a fish scratch it's self I put garlic on the food for 2 or 3 days.
From the advice from a high end LFS
owner. He said ICH was like the common cold, It's always around and you catch it when your immune system is compromised. Moving a sick fish to a QT tank adds to it's stress. Stress=Ich.
I've been told on this site that garlic won't cure ICH. Garlic Extreme works for me.