The CH in my 2,500-gallon in-ground spa has a tendency to creep upward over time (from about 250 to 375ppm in 3 months). I try to keep the level in the 200 - 400ppm range. Since we do not have hard water, I am curious what might be the cause. The chemistry is checked & balanced weekly & the spa has an auto cover. There is no scaling on the tile along the water line. I realize that this is not a critical issue; but I am just curious as to the cause. Thank you so much.