Ok. I've talked with Chem_Geek, and his thoughts confirmed mine. Add your second set of no-sun readings that pretty much eliminates possibilities other than the ones I mentioned.
But . . . Chem_Geek noticed some things I hadn't about the way you described your CYA testing.
So
#1 - K200x CYA testing is turbidimetric: the water gets CLOUDY when CYA is present.
#2 - You fill the small sample bottle to the first line with pool water and then add CYA reagent to the 2nd line.
#3 - Shake, wait 15 seconds, shake again, wait 45 seconds.
#4 - Then, carefully add this mix to the small CYA tube side of the kit, adding slowly, till the black dot disappears.
#5 - Stop adding when it disappears, and read your CYA level from the outside, matching water level with the marks.
BUT . . . if you the mixed sample water doesn't get cloudy, you have little or no CYA.
Ben
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