I use a python and treat the water as it is added to the tank. I have discus and I've never had any problems.Dump straight in
But a temp change like that could be very bad if you have ich in your tank right? I'm assuming you either managed to get fish that didn't have it or you've already been through a bout with it and have killed all of it off. Or am I just dead wrong about quick temp changes making fish more susceptible to diseases like ich?