I have a 10 gallon and a 5 gallon that get used as hospital / quarantine tanks. I learnt about Quarantine the hard way when 6 diamond tetras brought ich and a bacterial infection into my biggest tank and wiped it out in one week flat.
Now I ALWAYS quarantine, and if a fish looks ill it gets pulled out and put on it's own. Not just so I can treat it easier, but to save the rest of the tank too.
I just keep a couple of small filters running in my larger tanks, then if I need a QT tank I pull out a ready cycled filter and I'm ready to go.