Al, who was the owner of Rocky Mountain Discus for many years once told me that rain water will kill your fish. His exact words were "it will turn them into floaters". I will never forget it. I believe Al has since retired. He was in bad health and is no longer selling fish. Rain water does pick things up on its way from the sky to any container.... For example, if it hits the roof then into a gutter then into a container it could certainly be contaminated. I mean, most gutters are pretty nasty inside and roof surfaces and shingles contain all kinds of nastys. Even if the rain fell out of the sky right into your tank it could still be suspect. If your want pure water you are much better off with an RO filter in my opinion.