My city water is 166.(suburb of phila.) Every year it gradually goes up. If that holds true , next year this time it should be around 190. Source of water is a river that comes down from the mountains where steel and coal mining was done eons ago. I usually get about 165 gallons of ro/di @ .oo then another 55 gal.(not used in tank but used for cleaning and utility water)up to and including about .09, when it hits .10 i shut it off and change resin. I used to let it get to .15 and used it for water changes but since only using .oo water for water changes its helped with diatoms and cyano and coral color and pe. Im saving significant money by changing my own resin with the brs stuff.