Do you have a persistent reoccurring algae bloom? Do you live in Florida? If you answered yes...you most likely are experiencing a problem due to phosphates and or nitrates. Naturally occurring phosphates are the food that algae feeds on. In Florida we have Phosphate mines so in some areas the problem is worse than others. Phosphates are in just about everything from skin to fertilizers, etc... Prior to the 70's phosphates were used in laundry detergents and when it was known to cause algae blooms in our lakes and rivers...the phosphate was removed. Well, just like lakes and rivers...phosphates can enter your swimming pool from many sources, including...products used in your swimming pool by your service technician...Yep, that's right...it can come from pool chemicals unless the product specifically states it does not contain phosphates...you do not know. This is why I choose not to use products that contain phosphate unless there is no other choice. It only creates more problems and I don't know about you...but I prefer to minimize problems. I use Natural Chemistry...and believe me...it isn't cheap! So, if my prices seem a little high...it is because I am spending money on quality products.
Nitrates can come from acid rain, overhanging trees, fertilizer, birds, sweat, human waste, cosmetics,etc...
Both are common algae food...get rid of and do some preventative maintenance for phosphates and nitrates and you will be a happier pool owner.