Most of us don't store the change water. We just add Prime or another good dechlorinator to the half full tank, then use tap water to fill up the tank the rest of the way. The primary reason people used to age change water was to let the chlorine escape. But, now that most water companies use chloramine instead of chlorine, it take far too long to get rid of chlorine that way, and the ammonia is still left behind even if you do. Prime neutralizes both the chlorine and ammonia.