Yes, that’s what I was thinking too. Rain is mostly what is in my pond. It raining here now and just did a few of the basic tests on it such as Ammonia and Ph and Nitrate and I all checks out perfectly. As for chemicals, I have no way of testing. I would defiantly think its worth wild to check into. Yes, I can see, rainwater in the middle of summer in the Middle of a big city (New York, Toronto) can have some chemicals (acid rain) but for the most part, it should be fairly clean.