No tabs in the skimmer (not a good idea, anyway)... no liquid chlorine added... no cal hypo application... no SWG... for the last 2 months, and the FC continues to be 20 ppm?
Unless there is a mischievous neighbor or unruly teenager playing a joke on you (by adding chlorine at 3:00 AM), I would reexamine my test kit solutions or procedure.
Yes, very high levels of CYA will certainly slow down the process (more about CYA level, below) and the time of year means that the sun's UV rays are not reducing the chlorine very much but halogens dissipate over time. If you have a pool cover in place, remove it.
I, too, live in N. California and during late fall and winter chlorine levels take a long time to come down ... but not two months. My pool's temp is about 60 degrees F. right now. Chlorine demand is about 1/2 ppm per day. CYA = 40 ppm. No SWG operating, uncovered.
The CYA level in your pool probably started out to be much higher than 100 ppm (hard to accurately measure - have you tried diluting the sample with tap water 50/50 and multiplying result by 2?)
If you have a Taylor kit, read some of their articles about testing. If you're 100% sure your test products and process are correct and were desperate to reduce the FC, you could go the chemical route (thiosulfate) but I think you're mismeasuring and/or the CYA is crazy high. If this were my pool, I'd continue the controlled drains until CYA < 80 ppm.
Hope that helps.
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