The pool is 11 years old. I just switched from using Tri-Chlor to using liquid. From what I know, the previous owners used Tri-Chlor, as well, but I do not know if they ever had a pool service before that that might have used Cal-Hypo. In fact, they "taughtt" me how to do the pool when we moved in April 2012.
One more clue is that I run the solar heater especially during cooler months, and I have noticed immense evaporation and no rain for months earlier in the year. So the evaporation/refill theory makes sense.
So CH 160 fill is considered high, I suppose? So drain/refill would not be very effective? Except 160 is still way better than 450, of course. But, other than hoping for mostly rain and never having to use the hose, is there anything else I can do to keep CH from rising in the longer run?