I use a TDS meter on all the water I get. I have heard horror stories on using windmills or any of the other self serve water. I tend to go to my LFS and buy RO/DI water for 50 cents a gallon, or I will head to HEB to buy gallons of Distilled water. Both which have a TDS rating for the most part at 6, seen the distilled water go as high as 9. In a pinch, since I have sparkletts delivering water, I will use that, again only in a pinch and I dont have much other choice.