Pool Phosphates: What, Why, and How?

You may have heard about phosphates and phosphate removal products for swimming pools. In addition to proper chemical balance, controlling phosphate levels in the water is a critical aspect of preventing algae growth in pools. Here's some info about what phosphates are, why they are a problem and how to remove them from your pool.

What are Phosphates?

Phosphates are a compound of salts, like phosphorus and other mineral salts. Phosphorus is totally natural, and is an essential nutrient for human, animal, and plant life. It is one of the most common substances in our environment; occurring naturally in food, water, human and animals.

Phosphorous containing compounds are necessary components to our DNA and RNA, cellular membranes, teeth and bones and our metabolism. Plants need phosphorous compounds because they are vital to the photosynthesis process.

Phosphates are found in many products we use every day, including: energy drinks and colas, baking mix, toothpaste, some cleaning products and detergents (though use in those products is limited today), fertilizer and fire extinguishers.

Why are Phosphates Bad?

Right now you might be wondering how something so essential can cause problems in your pool. As I mentioned above, phosphates are necessary for photosynthesis, which, like all plants, is a process algae uses to grow. A large increase in phosphates allows algae and other plants to grow more rapidly.

phosphate molecule

In the 1950’s and 1960’s sodium phosphate was used often in household detergents to increase cleaning power. This added phosphates to the water supply and therefore dramatically increased phosphate levels in some lakes and streams. This unnatural rise in phosphate levels increased algae growth dramatically, causing many problems for aquatic life. As a result, efforts were made by the government and detergent manufacturers to reduce or eliminate the use of phosphates in household detergents and to remove them from the water during treatment processes.

This same idea can be applied on a much smaller scale to your swimming pool. Phosphate encourages algae to grow, by providing a good food source. Because they exist in so many natural and unnatural products, phosphates can enter your pool from many sources.

Some of the most common sources of phosphates in pools are: decaying plant matter, fertilizers, contaminated well water, acid rain, soil, ground water runoff, bird droppings, perspiration, urine, hair care products and cosmetics.

How do Phosphates enter the Pool?

  • Windblown fertilizer
  • Soaps, lotions, creams
  • Mulch or soil run-off
  • Bird droppings
  • Ducks and Dogs
  • Sweat, Cosmetics, Urine
  • Contaminated fill water
  • Leaf debris
  • Acid rain

How do I know if I have Phosphates in the Pool?

  • Difficulty maintaining a chlorine residual
  • Repetitive algae blooms
  • Test it ~ with a phosphate test strip.

Removing Phosphates from a Pool

  • Add a Phosphate remover containing Lanthanum
  • Remove and replace the water from a low-phosphate supply

Phosphates and Nitrates are in your pool, all the time, you can't see them or taste them, but they are there. To keep them at bay, keep your pool as clean as possible, and keep your water balanced, including a constant chlorine residual. Be careful with fertilizers as you apply them, and if your pool is prone to flooding from surrounding planter beds, do what you can to mitigate the possibility of overflowing into the pool.

How To Get Rid of Phosphates

buy test strips for testing swimming pool phosphate levels.

There are lots of different studies that seem to give some conflicting information about what level of phosphates in a pool is “too high”, but you might want to consider a phosphate removal product if your levels are over 100ppb (parts per billion), especially if you find that you have reoccurring algae problems. Use a phosphate test kit to quickly check your phosphate level. Make sure you do not have active algae in your pool when you test.

Phosphates are not always the cause of algae in a pool; if you have proper sanitizer levels and chemical balance, algae is unlikely to grow (unless phosphate levels are extremely high). You can liken phosphates to adding gasoline to a fire; gas didn’t start the fire, but it sure will spread quickly if added. Controlling your phosphate levels helps to give you some insurance that your water will not turn in to a swamp if your sanitizer levels dip for a couple of days.


In The Swim carries two types of phosphate removers, Natural Chemistry’s Phos Free and SeaKlear phosphate remover. Both chemicals remove phosphates from pools quickly by seeking them out and attaching to the phosphate molecule. As the attraction continues, the molecule increases in size, making removal possible with your pool filter.

It is important to remember that phosphate removal is a preventative measure, if you have active algae present in the pool, treat it with chlorine shock and algaecide first, then follow up with a phosphate remover to prevent re-occurrence. As algae die, they release phosphates in to the water, making it more likely that algae will return, creating a vicious cycle of expensive-to-treat algae blooms.

In The Swim makes every effort to provide accurate recommendations based upon current ANSI/APSP/ICC-5 2011 (R2022) standards, but codes and regulations change, and In The Swim assumes no liability for any omissions or errors in this article or the outcome of any project. You must always exercise reasonable caution, carefully read the label on all products, follow all product directions, follow any current codes and regulations that may apply, and consult with a licensed professional if in doubt about any procedures. In The Swim assumes no legal responsibility for your reliance or interpretation of the data contained herein, and makes no representations or warranties of any kind concerning the quality, safety, or suitability of the information, whether express or implied, including, without limitation, any implied warranties of merchantability or fitness for a particular purpose.