When I discover a fish with ich I lower salinity and raise temp up a bit and turn on my UV... I don't have a QT and I've never lost a fish to ich... I think trying to catch it, throwing it in a QT will just stress it out more which just bolsters ich, plus you're going to have to break your tank down to get that fish out...
On the other hand if the ich isn't eliminated it just gets worse and worse over time.