About a year ago, I decided to try cellulose fiber instead of DE in my filter. I've always had great experiences with DE, but I wanted to try something new (dumb move) and saw the cellulose fiber at Home Depot. Everything seemed fine after adding it. The water was not quite as clear as DE, but I wanted to give it a chancer so I left it in.
After a month or so, my CAT 1000 PH controller readings started experiencing large inaccurate swings in PH readings. I thought it was the sensor and replaced it with no change. I manually kept the PH in the ballpark for the past year, but was frustrated thinking that the PH controller was going bad after three years.
I did my annual pool maintenance today, and one of those steps was backwash and clean the filter and replace the cellulose and go back to DE as it kept the pool clearer. Once I cleaned the filter, re-assembled and added the DE, the PH system immediately started reading accurately again.
Seems strange, but possibly the cellulose fiber was causing problems with the PH system.
Sharing in the event you have a CAT 1000. Stay away from the cellulose...