I haven't visited for awhile, as you can see I am in the industry. This past year I have directed dozens of pool owners to this site because it is the single most informative and accurate site on the internet.
Here is my scenario- this is an indoor pool the water was replaced on 7/1/13. I balanced it for CAH at 300ppm. It has fill water coming in which measures 20 ppm. The pool evaporates approx. 1/4" per day. It's 20 x 30 x 4 (18,000 gal) therefore, I have calculated that it has essentially evaporated 85,500 gallons since 7/1/13. I believe, if my thinking is correct that I should have seen the CAH increase by 120 ppm bringing it up to about 420 ppm however it measures much closer to 620 ppm. The reason for my concern is because the pool does have signs of staining beginning to show. Prior to 7/1/13 it have major stain issues, it was at that time that I encouraged the customer to install a water treatment system which has all but eliminated any staining. We did drain and acid wash the pool prior to re-filling it on 7/1/13. The CAH when I took over the pool in 2012 was 850 with a TDS of 2000+. A co-worker had been handling the pool this year but I am back on it for the winter. I had been unaware of the issues beginning to creep back in until this fall. BTW- it is a bromine pool.
As a side issue this pool has a ever increasing PH problem. Every single week it takes 1 gallon of muriatic acid to bring it back into the 7.4-7.6 range. I have tried running the ALK as low as 70-80 and as high as 130-140 it seems to make no difference. I have also wondered if the degrading surface can possibly effect the PH.
So the over-all question is what effects can a degrading surface have on water chemistry, if any?