Tested with the t-100 test kit. The black dot was clear with the tube filled all the way.
We were having people over that night, so in the morning, I opened up the cover, tested FC, brought it up to the high twenties, then opened up the air valve to let it aerate, and get some sunlight during its two hour filter cycle.
When I came back and tested FC three hours later, it registered 3-FC.
That didn't seem right, so after some testing, I discovered that my Menards bleach had degraded down from 12 to something like 8, and the aforementioned CYA level.
I use the dichlor-then bleach method in my tub, and it had been about 2 months since the last water change. I had been adding dichlor weekly, about 5 ppm FC to keep levels up, but this apparently wasn't enough. Our tub sees heavy use, and is shocked up into the 20's FC weekly, at least.
I've read the CYA degradation thread, and although a lot of it is too technical for me, I've concluded that my CYA is being oxidized by chlorine.
After the low CYA test, I measured out one third cup of dichlor (around 30ppm CYA/FC) and added it in. Since then (about 3 weeks now), the chlorine demand has been much much lower. Half perhaps.
Also, PH rise has behaved differently. It just stays between 7, and 7.2. I haven't tested alk at all, although I did add 2 tablespoons of baking soda last week. Previously, PH always was on the rise, requiring about 1 TBSP dry acid around every 10 days or so, to keep it out of the red. (7.8-8)