What do you have in your tank as far as life goes? Inverts? Fish? What?
If you have just the fish, though other people in here will debate their personal preferences to you. My personal preference is if you have a QT, take the infested fish out, match the water temp with de-chlorinated water, put the fish into the fresh water, for about 3 minutes, keep a close eye on it during the process. If the guy is doing well, continue for another minute. This eradicates the C.irratans from the fish. Then place the fish into the QT with copper treatment (.11-.20ppm) for 2-3 weeks. A good procedure to follow is to maintain the copper level at the .11-.20ppm for an additional week after the last signs of the C.irritans were observed.
When treating with copper, there are two types, ionic and chelated. Ionic is a preferred type. Ensure whatever test kit you purchase to monitor the dosage is for the same type of copper (chelated or ionic). Also, you may treat with an anti-biotic during the treatment to prevent any secondary bacterial infections created by the tissue damage from the Ich.
Copper will remain in the tank so it is not receommended for your main tank. If you do not have a QT, now would be a good time to get one.
Remember, this is merely my own personal preference for the treatment of C.irritans.