High CYA does NOT reduce the effect of low pH on pool equipment; your installer is wrong.
BUT . . . with chlorine of 20.2 ppm (how did you test?), phenol red testing for pH will not be accurate. However, the Alkalinity = 7 ppm, when CYA = 84 ppm (bogus test result, BTW -- there is no field or dealer method of testing that can distinguish 80 ppm from 90 ppm CYA) suggest your REAL pH may be lower than 6.6.
Buy borax (better for your pool) or soda ash in the detergent section of MANY grocery and big box stores.
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