There are different theorys on Ich. I'm sure some folks will disagree with this, but here's the theory that I subscribe to;
Ick is present in a dormant stage all marine aquariums that are not chemically treated for said disease. When fish get stressed, their immune system is comprimised. This allows the ich to rear it's ugly head. Therefore, the key to keeping ich in check is to keep the fish from getting stressed. It also seems that when one fish in a tank developes an outbreak, it is easier to spred to other fish that may have borderline immune systems.
As far as treatment goes, there are few that are reef safe. Feeding the fish garlic, which is high in antioxidents and vitamine C IMO,seems to be the safest and most widely used among reef keepers.
As far as other treatments, some of them are actually more deadly than the ich itself.
Like I said, this is just my opinion, from my experience.