If you keep it at a QT tank, just remember that it won't stay cycled forever without a source of ammonia (i.e. fish in the tank). You can get around that by just tossing in some flakes of food each day and occasionally using the gravel vac to clean them out. Pure ammonia would work too if you can find some. You probably won't need more then a drop or two a day to keep the level constant. It's just to have enough to feed the good bacteria and keep them alive. Without a food source they will die off and you will have to cycle the tank all over again. Just a heads up.
Sure anything that will product ammonia will work. Just watch out if you use the tank to treat a sick fish as meds can be killers to others in the tank. So be sure to more whatever you keep in there out before using as a hospital tank or a quarantine tank.