This doesn't make sense to me. I moved to a house where previous owner exclusively used tablets for many years. Sure, CYA ended up being somewhere between 240-300 but shouldn't it have gone up to much higher number?

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After the drain and refill, CYA is now somewhere between 70 and 80.
Hopefully this will make it easier for me to keep algaes away. I plan to keep chlorine level between 5 and 10.