If there are fish in the tank from when there was ich, than there will be ich when you introduce another fish. ALL fish must be out of the display tank for a minimum of 1 month. You should QT the fish in your system for a minimum of 1 month than you can be sure there is no ich. Just because you can't see it on your current fish, doesn't mean that it is gone. The "appearance" of a healthy fish is why ich and other ailments get into our tanks in the first place. Just like humans, people walk around with all kinds of ailments without any symptoms. To be sure, you must QT everything.