Hrm, I would think that rain water would lack essential minerals and tend to be slightly, or much more acidic depending on the air pollution where you live. In a natural setting this rain water has a chance to be filtered through the roots of countless plants and picks up minerals from sedimentary rocks--personally, I would just buy more dechlorinator. There are very cost effective ways to keep up your supply of dechlorinator, for instance, you could buy it in powdered form and make up your own concentration of dechlorinator.