Ich (Cryptocaryon irritans) is a living creature. It can't appear magically in every tank (though it can sometimes feel that way!) and if you're rigorous with quarantine, you can have a tank with no ich. (Which means you can also have things like achilles tangs!) It can be present on a fish's gills and enter your display, though the fish shows no symptoms. It can be encysted on a frag plug, piece of liverock, or a shell - almost anything hard and wet, and enter your display when you've added no fish. If you're keeping your seawater in a fishless barrel for 76 days, it should be fine (from an ich standpoint, at least) to put into your tank.
If your fish had ich in your display tank, and got over it - and they weren't removed for a 76 day fallow period - then ich is probably still in your tank and on your fish, just at a low level. That's what happened to me when I started out. Bought fish, placed in tank. Ich outbreak! Most fish recovered. Bought new fish, only to see ich move on the vulnerable newcomers as soon as they entered the tank, killing fish after fish. I wound up pulling all the fish for a fallow period, and have quarantined all fish since.
To date, I know of zero reef-safe treatment options, I'm afraid ... if one is ever found, I'm sure it'll be announced with a trumpet fanfare! Using other treatments, like copper, can be a challenge in your display. Liverock absorbs copper, bringing your copper level below that which would kill the ich - and can then _leach_ copper into the water at some later date, stressing or killing creatures you _want_ in your tank.
~Bruce