My pool, located in San Diego, was resurfaced last year with a medium grey plaster. After start up the pool company set the CYA to 60 PPM. Not knowing better, I used tablets for a few months and my CYA rose 80 PPM. On 2 separate occasions this summer algae growth has forced me to shock the pool following the "pool school" instructions using bleach and muriatic acid. However, both times the algae returned within a few weeks. Being at the upper end of the CYA spectrum, I suspected that this might be impeding my efforts. 10 days ago I completed the shock process and passed the overnight test. The next day I treated with polyquat 60 for 24 hrs. I then backwashed the filter and drained 25% of the water to get my CYA down to 60 PPM. Since then I've checked my levels daily and kept my FC between 5 and 9 PPM. PH has been between 7.4 and 7.8. All testing is done with the recommended Taylor 2006 kit. Now 10 days later I see algae forming on the vertical edges of the stairs in the shallow end. This is usually where it starts and then it spreads around the entire surface of the pool. It appears to be green, but the true color could be skewed by the grey plaster. Oddly it only grows on plaster, never the vacuum or the vacuum hose which remain in the pool 24/7. The water itself remains clear.
I'm at a loss as to how algae can be taking root in a pool with ideal levels of free chlorine and the following parameters.
DE filter runs 9 hrs / day
Alk = 190
FC = 5-9
CC = 0
CYA = 60
pH = 7.4 - 7.8
Anyone have any ideas as to what the problem might be? I am guilty of not brushing my pool. In fact in the past 10 days I did not run the vacuum or brush the sides. Could this be enough to seed the problem?