Tomonts land on surfaces and can come in on rock, corals, frags, snails, etc. Theronts are in the water and can be on a droplet of water on rock, corals, frags, snails, etc. Theronts can be filtered, but there is no chance that you get them all.
The only way to make sure that you have no ich in your tank is to do thereputic copper on every fish and then have a coral and invert QT that is fishless where you made no additions while the tank goes through the fallow period. My guess is that probably less than 1% of folks actually do this. Most folks that do copper on their fish do not QT inverts or corals so they probably have ich in their tanks and the fish are just strong enough to fight it off - they just think that they are ich free. Most of the general public just roll the dice and the fish that live get immunity or are just tough.
The only way to make sure that you have no ich in your tank is to do thereputic copper on every fish and then have a coral and invert QT that is fishless where you made no additions while the tank goes through the fallow period. My guess is that probably less than 1% of folks actually do this. Most folks that do copper on their fish do not QT inverts or corals so they probably have ich in their tanks and the fish are just strong enough to fight it off - they just think that they are ich free. Most of the general public just roll the dice and the fish that live get immunity or are just tough.