Thanks for the advice. I will try to find CYA locally from a different store than Leslie's, and order online if no luck.

The bleach is doing the job at 10 to 15 ppm, because the water is now clear gray instead of green, and I can mostly see the bottom. The sand filter hasn't clogged yet, so I must be backwashing it often enough. The pH is 7.0 (or lower, I can't tell with my Taylor kit). I'll adjust using borax as if I were trying to adjust from 7 to 7.2 and see what happens. I'm off to brush the sides and clean the bottom.