Get a QT and treat with either copper or Hyposalinity. Nothing's else cures Ich. Also, leave your DT fallow for 6 weeks. That allows time for the Ich in there to die off due to not having a fish host.
There are several sites that talk about Marine Ich treatments, what works, myths and misunderstandings, etc. Read up and understand the enemy.
QT future additions prior to adding to your DT and you should never have to deal with Ich in your DT again. This is especially nice once you have several fish in there.
Not sure about the other fish...I've seen fish disappear for days and suddenly they are there again. Did you check around the tank for jumpers?