The success of your treatment depends entirely on what kind of illness the fish has. Look up some pics of various diseases. Does it look like salt sprinkled on the fish? Ich. Does it look like cysts with velvet? Maybe Velvet. Does it look like cysts with slime? Perhaps Brooklynella. The Disease forum on this website is fantastic and the people there will be able to point you in the right direction.
Generally speaking, you always want to treat your fish in a quarantine tank instead of the main tank. (Unless, of course, you've got a fish only tank with no live rock, no inverts, and rampant ich... but that's about all.) That's because the treatments can be highly toxic to your live rock and inverts. For example, with ich, you treat the fish with hypo in a QT for a few weeks. In that time, the ich in the fishless display tank will die off, but the inverts will be okay since you didn't mess with the salinity. And you should NEVER put copper in your display tank. Copper is toxic to inverts, and will bond to absolutely everything including rocks. That means even after changing the water, that crushed coral substrate will continue to release copper into the water and kill off your inverts as well as make your live rock into dead rock
So in short, try to narrow down your disease and then hop over to the disease and treatment forums to figure out where to go from there. And remember ALWAYS QUARANTINE both new and sick fish. It's the cheapest form of preventive maintenance!!
Goldie