Treat the fish with copper solution in a separate hospital tank, and leave the main display tank without fish for 6 weeks, which is the life cycle of ich. Technically once in the tank its nearly impossibly to get rid of it but letting your tank cycle should get most of it.
There's lots and lots of great info on treating Ich on WetWebMedia.com
just go there and search for "ich" and uncle bob fenner will take care of you.