I don't have a tank permanently set up as a quarantine tank, it sits empty. What I like to have is a spare filter (either sponge or HOB like an AquaClear) running on an exisiting tank. If a fish needs quarantining, just fill the spare tank with water from the existing tank, transfer the filter over and add a heaterstat (one I know is accurate and works properly).
You can't leave a tank running with water, heater and filter without fish to keep the bacteria fed in the filter.
Of course, if you have had disease or used medications, do not put the spare filter back on the main tank, when finished, without cleaning it thoroughly first!