no matter what you will always have some form of ich in your tank, beleive it or not. It can lie dormant at the bottom of the tank for awhile, it is when your fish become stressed and succeptible to parasite infection that it appears, but it will always be in your tank. At least this is my understanding of this little pain in the arse. Anywho, just monitor your fish and feed them, and keep them happy and healthy and you should rarely encounter this problem!
good luck
jon