Yeah you do not need to run a QT tank all the time. Not QTing fish is terrible though and there is no excuse for it. Its not fair to your fish!
I have my 40G QT set up with a canister filter, heater, small clip on light but I usually just use sunlight and PVC for hiding in.
I keep the canister filters sponge and carbon in my sump until I need the QT. When I need it, I use 40G of water from my display tank and the media from the sump and set up the QT. Qt new fish for 3 weeks and if nothing comes up, I add them to the display along with the sponge from the filter. I drain the QT and put it away. If they do get ich in QT, then I go with hypo usually for 8 weeks and when its done, put the fish in the display BUT this time, sterilize the sponge to make sure Im not putting parasites into my display. Then you just have to let the sponge sit in the sump for a few weeks again until bacteria colonize it. Very easy and worth it. Personally, I QT anything new, not just fish. I never get disease in my tank since doing this. Before this, I killed more fish than I care to admit because I never QT'd. Very sad when I look back
