You only need to treat the effected fish. The treatment is formalin bathes, and the procedure is detailed in the FAQ Thread at the top of this forum.
Not having to deal with this hassle is the reason why setting up a QT is so very important, and using it for all new fish. Fish will have parasites. There is no avoid it. IMO, if you have a reef tank, there is no choice but to QT all incoming fish.