Just a comment to add to Watermom's advice....the shock level for a CYA of 70 is actually 20 ppm, and you won't be able to get an accurate pH number with your chlorine that high, unless you're using a K-2006 kit. If that's the case, then you can work on the pH and shocking together. If you're using the regular OTO kit like you get at WalMart, then you'll get falsely high pH readings when your chlorine is over about 10 ppm. So....if that's the case, I would add your Borax, make sure you're over pH 7.0, (because pH below 7.0 can damage your liner) and THEN go ahead and shock your pool.
Janet
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