Quote Originally Posted by lisak View Post
21 is at or above shock level, correct?
Yes, your shock level for a CYA of 45 is 15 ppm. See the "Best Guess Chart" here http://www.poolforum.com/pf2/showthread.php?t=365 that we use as a guideline for shock/chlorine levels, based on your current CYA levels.

Janet