Took water to test in a pool store I go to, they have computerised testing.
Amongst other things the results show 500ppb phosphates. I do not use any lawn chemistry, so the explanation suggested by the store guy was "dead algae"I. (I had light algae bloom yesterday (first in the two recent years) that I quickly squished with keeping the pool at 22ppm chlorine for 24hrs.)
Is this a plausible reason and what is the best remedy to remove the phosphates (I know what the store will sell me).