Water Tanks, page-12

You are missing my point.
Connect the filtration system I mentioned to your RWT and then you have the very best water because all of the impurities that flow into it from the roof and gutters and downpipes are fixed.
Fit the same system to your town water on your side of the meter then all of the additives that the water corporation put in are fixed also. Don't be fooled by the old adage that rain water off your roof is pure and clean.
I have been brought up with rainwater as the main supply over a period of more than sixty years so have experienced the pitfalls and belly aches prior to the modern day comprehensive filtration systems becoming available.