I have a QT tank after I experienced a total disaster. When I first started in thius hobby just under two years ago I was having an easy time of it, no fish deaths and everything was just peachy. Then I bought a coral beauty angel who brought in ich and ended up killing every fish in the tank. I couldnt treat the tank with copper because of the inverts.
I then setup a 29 gallon QT tank and all fish get QT'ed for a minimum of 4 weeks. I have only had one fish die in QT of mysterious circumstances. If he did die of disease he could have easily spread it to my other fish. I have not had an outbreak of ick or any other disease since starting the QT. I also run the QT at the same salinity as the display tank (~1.025) to keep down the stress when moving the animals from QT to the display.
My QT tank is simple. A 29 gallon tank with an emperor bio-wheel filter, a heater, a single rock and PVC pipes and sand on the bottom. Most people wouldnt include the sand but this was a converted display tank.
I will NEVER go without a QT tank ever again.