It appears, after some furthur research on the subjec,t that the quality of the rainwater in a given locale varies depending on enumerous factors. Jet-Stream /Originating Altitude / Originating or evaporative locale, etc... The quote below is from science daily and I have included a link to their site/page with similar topics... Apparently California is being pollutted by Mercury from ASIA....
"— Industrial emissions in Asia are a major source of mercury in rainwater that falls along the California coast, according to a new study by researchers at the University of California, Santa Cruz"
My opinion would be to steer cleer of rainwater for aquariums, we dont culture the specific bacterial strains indoors that naturally develop with slow introduction of various chemical compounds in our outdoor ponds... Just an opinion..