I recently purchased a home with a 3 year old pool. I have recurring algae issues and am not sure where to go. The pool guys keep having me add shock and phos prevent. When the water is tested it has shown a 0 chlorine level. My Pool Pilot Digital Nano has the check system light lit (solid red, not flashing) and there is no error message displayed. We cleaned the filter and they visually checked the cell and saw no issue. Any thoughts?
Pool Pilot Digital Nano model#75041
Pool Capacity 5500 gals
Concrete
Salt is showing as 3200

P.S. It was crystal clear when I moved in.