Have had recurring minor green algae problems ever since we built the pool 30 years ago. Two years ago changed to salt chlorine generator from Bromine. Have always tried to maintain proper levels but have not been able to get residual chlorine above .5. Should be 3-5 ppm. Two years ago Local supply store put in computer water analyzer that now tests for phosphates. Have been 3-5000 ppm, should be less than 500ppm. Have been using Phos Free but not putting dent in phosphate level and requires several back washes during season. Found product called Phosfloc made by same company (Natural Chemistry) but not readily available at local stores. Found on internet and recently used. Filter media must be out of tank or filter must be completely bypassed. Put in four small tubs (40,000 gal pool) +/- $80.00. circulated for 8 hours and shut off. Settles into cloudy mush in bottom of pool, vacuum out next day onto grass without going through filter. Phosphates now 400ppm. Should resolve algae, and low residual chlorine problems, will see. Phosphates are put in public water to help pipes keep from corroding, most people don't test for them, high phosphates create all sorts of issues.