There are two schools of thought on water.
One says use only coolant and distilled water.
The other says that only tap water should be used - distilled water is supposedly corrosive.
I've seen heated debates on this subject here and on the Ritter-Easley list.
It's my opinion that grocery-store-bought distilled water is not a problem. There are lab grades of water that supposedly eat thru metal pretty quick. I've often wondered if the .59 cent variety has been confused with the high $$$$ lab type.
I've used disitlled water and coolant for many years and the inside of my radiators stay very clean.
My 2 cents.