Mark had poured in shock—bags of the stuff. But his pH was off the charts (8.2, he later learned). The chlorine was present but useless, like a guard dog locked in a soundproof room. The cheat sheet explained: Total Alkalinity is the mattress; pH is the sleeper. Adjust the mattress first (80–120 ppm), then tuck in the pH. He added sodium bisulfate (pH down) and felt the water sigh.
Here are some common pool chemicals and their functions: pool chemical cheat sheet