Even though you can't see the ich on the other fish, it could still be(very likely) in the their gills or in the sand/substrate, floating in the water or laying dormant on the rock, corals, inverts, etc waiting to hatch and then swim around to find a fish to live on and infest. That is why it is a good idea to leave the tank fallow of fish for no less than 4-6 weeks after the last spot of ich is seen on the fish in QT and why it is also a good idea to QT anything wet prior to putting anything into the main tank after a purchase or bringing a new specimen home.
And you can forget about all those "reef safe" bottles of water the LFS try to sell you. They have never been proven to cure ich in a reef tank as of yet. The three methods I mentioned above ,as well as the few others mentioned, are the only PROVEN methods of killing ich.