if you are using this water to replace used up elements like calcium, i would not store it for any extended period. calcium precipitates out fairly quickly, so if you wait long enough, your calcium level will be kaput in your change water and your rubbermaid container will have a crust on it. i would not keep it for very long in that case. if this is just a fowlr, then you can keep it indefinitely, as long as you keep it moving (a small power head should do).