I have a 25,000 gallon salt water pool with a solid electric cover, solar heat and a 60 square foot DE filter. Last year never had an issue with the pool. I kept the SWG at 20% generation all summer, and everything was stable.
Because of the electric cover, which I keep closed at nights and when not at home, I never have an issue with evaporation or rain water contamination, so never have to add/subtract water, except when I backwash the filter. The pool is controlled by a Pentair Intellitouch, so the temperature from the solar is kept pretty constant, so I don't have wild temperature swings.
Over the past 3 weeks I cannot raise my FC above 0 and my CYA above 40. I use the TF-100 test kit, along with a LaMotte digital pH and TDS meter. I have been running the SWG generator at 100% for 3 weeks (a lot of chlorine bubble activity in the SWG cell, so I know the cell is working), adding almost one gallon of bleach per night for 2 weeks straight (15 gallons of 6% Chlorox regular bleach added total), 4 pounds of di-chlor shock, and adding 6 pounds of CYA per week (18 pounds 100% pure CYA total). Water is crystal clear.
Here are my latest readings from the TF-100 and other methods:
FC - 0.0 (tested using TF-100 chlorine drop test, verified with test strips)
CC - 4.0 (tested using TF-100 chlorine drop test, TC verified with test strips and Taylor pH tester)
pH - 7.1 (tested using Taylor test kit and LaMotte pH tester - a bit low, but I just brought the TA down)
TA - 110 (tested with TF-100 TA test)
CH - 360 (tested with TF-100 CH test)
CYA - 40 (tested with TF-100 CYA test, verified with test strips)
Copper - 0.3 ppm (tested with test strips)
Iron - 0.0 ppm (tested with test strips)
Phosphates - 1000 bbp (tested with test strips)
Salt - 3800 ppm (tested with LaMotte TDS digital tester, calibrated at the beginning of the pool season)
Why can I not get my FC to budge all of a sudden, even with the SWG at 100% and having added 15 gallons of bleach? Next, why can't I get my CYA to budge, even after 18 pounds of pure CYA? My pH was at 7.8, and my alkalinity at 140, so I added muriatic acid last weekend, which got my TA and pH down, but still can't control FC or CYA.
What do I need to do to adjust, and in what order? Any help is appreciated, as I am getting quite frustrated this time around.
Also, other than adding the shock this one time, the only chemicals I add to the pool are bleach, muriatic acid, cyuranic acid, and iron free softener salt. I don't use any algacide, pool additives, etc. Thank you.