has told me that if you have ich in your tank and your fish are stressed then they will get it. If they are comfortable and eat well then they will have a very good chance of not getting it. Garlic and temperature increases are just preventative measures according to him. He told me to just do water chages to limit the amount of ich in the tank.
The idea that if a fish is unstressed and they won't get it seemed opposite for me, IE my yellow tang was very nervous and would hid all the time and it got ich but my percula was very hardy and didn't seem stressed at all (I couldn't ask them how they felt, but they seemed that way
) and it got ich. I think it is about doing the right things but mostly LUCK!
I will see the results when I can put my fish back into my main tank.