I might do it with circulation and aeration if it never wsa exposed to live rock, sand fish or corals.........but odds are I would dump it and mix new up when I needed it. I am kind of thrifty, but I see no point or situation I can conceive of having to save used water. I save my old water just ong enough to make sure I don't have a spill when doing a water change and once the tank is topped off the old water gets pitched.