The biggest worry is it sitting against the liner for days in a big pile. You are plaster though, so I doubt it would have any effect even if this is the case. Just brush that area really well for the next few days 1-2x to be sure to keep it moving around.
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Depends, what is your pH right now?
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If you are at/under 6.8, I would stay out. Under 7.2, I wouldnt open my eyes underwater.
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BTW, CYA takes a week to fully measure on tests. Maybe even longer for you since you just threw it in.
I would use whatever CYA level you targeted as your current level, regardless of what your CYA test reads until that point. So bump up your FC to the target associated with a CYA level of 70-80.