Today's OTO test results:

Poolwater: pH 7.8; Cl ~8 ppm; alkalinity ~80 ppm; CYA estimated at 40 ppm after adding 4 lbs. of stabilizer for 21K gals. (assuming zero CYA level to begin with as only regular liquid chlorine was used for the past 2+ years)
Tapwater: pH 8.2; Cl < 0.5 ppm; alkalinity ~30 ppm
Water appearance: white with suspended solids, somewhat better than yesterday

I'm definitely getting a FAS-DPD test kit as recommended. Has anyone used any of the Lamotte Color Q test kits? These use a digital colorimeter tester than seems to be easier (and perhaps more accurate) than using color charts.