If you have never seen it in your tank, then you probably bought a fish with ich. I never had to deal with ich on my PBT. But when I got it in my 240, I had no QT, I just fed well and added vitamins to the food. Everybody lost the visible signs of it, and have not had any breakouts since. Now that you have seen it, you know that your whole tank has it. You can either setup a QT and put all the fish in it like Woodstock did, or you can deal with it in the tank. By deal with it I mean you can try the medications intended for ich, or you can do what I did. Feed well, and rely on the fish's immune system.
Good Luck
~Michael