Phosphate build up

[Frank]

Where does the excessively high phosphate build up come from late in the season?

[chem geek]

It can come from fill water so from evaporation and refill as many water utilities use phosphates as a corrosion inhibitor. My water has about 400 ppb phosphates in it (I actually measure more, but these are the numbers my water utility gives me).

Another source of phosphates is fertilizer so soil is generally high in phosphates (and nitrates) and can get blown into the pool.

Nevertheless, even high phosphate levels are not a problem if you maintain sufficient Free Chlorine (FC) relative to the Cyanuric Acid (CYA) level. My own pool now has 2000-3000 ppb phosphates yet does not get algae because I keep a minimum FC of at least 7.5% of the CYA level.

Phosphate removers are expensive, but should be seen more like insurance similar to using PolyQuat 60 algaecide. You can certainly use them if you want to, but they are not necessary if you maintain proper chlorine levels relative to CYA. If you feel you cannot maintain such levels, then the use of an algaecide or phosphate remover are (more costly) options.

Richard