ORTHO-PHOSPHATES (which this kit measures) are the limiting nutrient for algae growth. Without phosphates algae cannot grow, so controlling phosphates in a pool is very important. Phosphates occur everywhere in the environment. Sources include; runoff, leaves, vegetation, fertilizers, rain, dust, people, animals, and many other sources.

Fortunately, phosphates can be easily controlled by:

Testing frequently, especially in the spring and fall to monitor phosphate levels.
Removing phosphates (depending on range) and keeping phosphates below 100 ppb.
Use Pool Perfect+PHOSfree weekly following label instructions to maintain phosphate levels near zero.
 

Phosphate test kit for professional use,
2 Vial Containers with 50 strips per vial plus one test tube.