I am considering a phosphate remover to help prevent algae growth. (I have read many of the threads here on this topic, and understand that adequate FC levels will prevent algae.) Two specific questions.
I believe that my pool has very low phosphates, but I want to make sure I haven't overlooked an obvious non-trivial source. I contacted my water utility and found that phosphates in the water supply are below detectable levels (13 ppb). So no major source there. No agriculture or industry nearby (I live in Berkeley, at the foot of the hills, about 1.5 miles from the bay). Pool is covered 18 hours/day. Very little organic debris in the pool. Swimmers are clean. Clothes washer detergent has no phosphates, so no residue there. Small amounts of phosphate presumably from soil, organic matter blow in. Had algae once when FC went to zero by mistake. So I'm thinking I have very little phosphate, and it is the limiting nutrient.
Question #1: Is there some potentially non-trivial source of phosphate that I could be overlooking?
Question #2: If my phosphates are as low as I suspect, can i use less than the recommended amount of Orenda PR10000 (to save money) without reducing its effectiveness?