It could well mean that. It could also mean that my first test, before I even hooked up the pump, means that lighter, softer rainwater was on top and the older pool water was down below. Now it's mixed up.

Now I would be surprised if the amount of Di-Chlor I added raised CYA from 0 to 40--it was two DE cup measures. There shouldn't be anything close to that much CYA in the Di-Chlor

Meanwhile, NOTHING I have added could cause T/A to go from 20 to 80 overnight. Di-Chlor is acidic, so pH went down, which causes T/A to go down. I added nothing else.

So I'm convinced it was layered water that gave me those numbers.

CYA takes a long time to dissolve when it's in its own form. I would expect Di-Chlor to change CYA much faster since it dissolves within minutes to an hour, rather than 2 to 7 days.

We'll see where it goes, but I'm not worried. I still plan to switch now to bleach.