If your fish got Ich when in the 55 gal... then you can be sure that Ich will be in that tank, and you'll have to treat it entirely. Think of Ich as a fish-cold. If one of your fish living in a community tank had it, its really possible another one might catch it as well. The snails, as far as I know, don't get Ich. It only affects fish. But, the medicines you might be using to treat the tank will affect them (the snails), so you should put them in a QT place. You really should wait a week before introducing your fish back to his tank, in 2 or 3 days nothing happens, as I said before, to kill the entire cycle of Ich you must treat and wait 1 week.