<Distilled water is a known, tap water, who knows? I always use
distilled when mixing up coolant.>

In my case, the tap water is known-known to be crap! It comes out of the tap black, the ceramic tile in my shower and bath is etched from this horrible water, and my faucets all have a heavy calcium build up. That is why I said I'd use distilled water when I change out the coolant.    Jeff