Phosphates In Pool Water: What They Won’t Tell You



For pool owners who naturally want to control algae in their pools, taking steps to control phosphates just makes sense. Or does it?

Most people started to pay attention to phosphates in the 70s and 80s, when news of algae blooms and their effect on drinking water started to hit the news. There was a sudden awareness that phosphates are in everything.

Today I’ll be looking at the facts behind phosphates in pools, and what you really need to do to keep your pool water healthy and algae-free.

What Are Phosphates? 

Phosphates are a class of organic chemicals derived from phosphoric acid. They are a naturally occurring form of the element phosphorous, and are widespread in almost every organism and biological system on earth. 

They play a role in the growth rate of organisms, and phosphate ore is mined for all kinds of agricultural and industrial purposes. 

Where Do Phosphates Come From? 

Since phosphates are so abundant and used in such a wide range of applications, it can “come from” almost anywhere. In a swimming pool, phosphates can accumulate from four primary sources:  

  1. Swimmers: human sweat, skin and hair products, and detergents used in towels or bathing suits, all can contribute to the level of phosphates in a pool. 
  2. Water runoff: fertilizers and garden care products that are used around a pool may add phosphates to pool water
  3. Organic matter: leaves, twigs, algae, and other microorganisms may contribute phosphates to a pool
  4. Pool water itself: many municipalities add some phosphates to the water supply, to reduce levels of lead and copper. In addition, some pool cleaners and chemicals, along with stain and scale removers and stabilizers, may contain phosphates.

In other words, since they are so widespread in nature and used in so many ways, any pool water will always have some amount of phosphates in it. Phosphates are here to stay. 

What do Phosphates do? 

At low levels, and in pools without an algae problem, phosphates don’t do anything to affect the water quality in your pool. Phosphates don’t interact with chlorine, and naturally bind to calcium and other minerals, so some level of phosphates may actually help to soften and improve pool water. 

At levels above 1000 ppb, and in pools with an algae problem, phosphates can make a bad situation worse. They contribute to algae growth by feeding the algae, and, when the algae organism dies, it releases more phosphates into the water, feeding more new algae and creating an unhealthy cycle. 

In nature, this is called an “algae bloom”, and it’s why phosphates have such a bad reputation. 

Are Phosphates a Problem? 

The short answer is no, phosphates alone really aren’t a problem in pool water. Algae is the problem, and most pool owners employ other methods to control algae. 

How do You Test for Phosphates? 

Because phosphates aren’t really a water quality problem, most pool kits do not include phosphate level tests. However, phosphate test kits are widely available at pool supply shops, and are inexpensive and easy to use if you want to test your phosphate levels. 

Is Removing Phosphates Even Worth It? 

Generally speaking, most pool owners don’t need to test for or control phosphate levels in their pools. If your pool water is otherwise healthy and balanced, phosphates won’t affect water quality. 

If you have an algae problem, an algaecide is a much more direct and effective method of correcting it, than independently testing and trying to control phosphate levels. 

How do You Control Phosphate Levels in a Pool? 

All that being said, you can take steps to manage phosphate levels in your pool. Here are some of the best ways to manage phosphates: 

  • Skim regularly. Removing dead and decaying leaves and organic matter from a pool will remove one of the key sources of phosphates, and make your pool more attractive at the same time. 
  • Clean your filters. Like skimming, cleaning the pool filters removes organic matter and microorganisms that may be adding phosphates to your pool. 
  • Keep your pool balanced, and control your calcium levels. Calcium should be one of the values you regularly test and monitor when maintaining a pool. Calcium is a positive ion that bonds with phosphorous, a negative ion. This is why phosphates can soften pool water, but also why low calcium levels can create high phosphate levels. If you balance your pool for calcium, it will naturally control phosphates. And maintaining the correct pH, alkalinity, and chlorine levels will help to prevent algae. 
  • Control algae with an algaecide. Since all this talk about phosphates is really an attempt to control algae, just go to the heart of the problem and regularly use a good algaecide. 

As you can tell, these phosphate removal methods are (or should be) part of your pool maintenance routine anyway. Taking good care of your pool will naturally control the amount of phosphates.  

And, in case it needs to be said, you don’t need to use a phosphate remover. In fact, phosphate removers may contain chemicals that are even less desirable than phosphates themselves. They’re not necessary, and not even the best way to control phosphates. 


There are a lot of myths and mis-information about phosphates and what they do in a pool. But the bottom line is that some amount of phosphates is inevitable, and it’s better to focus on keeping your pool water balanced and algae-free than worry about phosphate levels. 

Since you have so many other things to test and measure and monitor in a pool, the good news is that you can safely cross this one off the list. 

Categories: Pool Care, Pool Chemistry