I agree with Watermom, pH and chlorine first, but it won't hurt to go ahead and add your CYA, just don't broadcast it like the label says, and if you add it through the skimmer into a sand filter, don't backwash the filter or retest for CYA for at least 4-5 days because it takes a long time for it to dissolve. Until you have CYA in the water, you're gonna have to make frequent chlorine additions because the sun will eat up a lot of your chlorine, so the sooner you can have it in the pool the better.
Janet
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