I've been using roof run-off rainwater for years with absolutely no ill effects. Water runs off the metal roof into collection cisterns, filtered through a 3-stage unit, and into aerated storage tanks in the hatchery.
BTW, all the pollution warners should check the variability of municipal water throughout the year. Reservoirs are filled through rainwater, rainwater runoff, and creek, river, or groundwater. A reservoir's chemistry is susceptable to changes from run-off, fire pollutants (ash, resins, retardants, etc.), drought, boating, inlet pollutants, etc. Then, there's the changes from treatment plant chemicals which vary depending on lab testing for biologicals.
And, well-water is no safer. Run-off, chemistry of bedrock minerals, vug gases, origin of groundwater, etc. will greatly effect what's pumped up from a well.
It all comes down to knowing what you're bringing into your house for everyone in the house (including your pets). I get a quarterly water report about my municipal water and check the utility website regularly if I detect or suspect a difference in my tap water. BTW, my municipal water goes through a 5-stage filter before going into the storage tanks.