Rain water was cleaner back then and shouldn't contain many impurities unless you live in an industrial area or have lots of traffic driving past.Back in the Olden Days, when it were all trees round here, etc., people would keep a large open-topped barrel or tub, to collect rainwater for the garden. Fishkeepers learned that this water was actually mature and suitable for their aquariums, when heavily chlorinated water from their taps was not. (Life before water conditioners, remember). Such tubs didn't have plants and it was the simple process of leaving the water to sit awhile that removed the unhealthy (for fish) elements to dissipate.