In my case the pool had not run in at least a year, and likely longer and the CYA was at 0 on my test strips, two tests at pinch a penny and finally on my test kit too. I had picked up some dichlor shock at an ace hardware just up the street since it was at $2 a pound. Now that my test kit is showing the CYA right about where it belongs the other pound I hadn't used will likely sit around for awhile. My Ph has been on the low end all along as well and I think adding the dichlor drove it down a bit (pool math said it would so it wasn't a surprise). The hard question is that while normally Ph is ignored during a slam, but at the same time it also normally is driven up by the chlorine and drops back at the end of the slam on its own.
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