You will end up getting a zillion suggestions on how to treat an ich outbreak in your tank. The two schools of thought are these...
1) ich is always in your tank, and unhealthy conditions cause it to actually take hold on your fish. Otherwise their immune system fights it off naturally.
2) ich is not always in your tank, and should you have an infected fish in your tank you should remove all of your fish for at least one ich life cycle to remove it from your tank.
Neither theory has been totally proven. I tend to go with theory #1 here, but that's beside the point. This is what you should do regardless of what you believe:
Take your infected fish out of your main tank and put them into a QT tank. Treat the ich using either a copper treatment (make sure you're testing the copper levels to keep them where you want them), or treat it with the hyposalinity method.
Here is what you should do only if you believe theory #2:
Move all of your fish to the QT tank and treat them. Keep the fish out of your main tank for 4-6 weeks. After that you can return them to your main tank.
Could someone please link the appropriate articles for hyposalinity and QT tanks for him? 8)