Phosphate: Why your pool keeps turning green

Green pool phosphate example
Phosphate is an incredible plant food, but in swimming pools it accelerates algae growth, turning pools green at the drop of a hat.

What Is Phosphate?

Phosphate is an organic compound that’s found in all kinds of products you use every day. For example, most hair conditioners have high levels of phosphate in them. Lawn and plant feed also have very high levels of phosphate. In fact, one of the main pollutants from farming “run-off” is the phosphate from fertilizing crops.

As far as plant food goes then, phosphate’s one of the best. Think about it though, algae is a plant. If run-off from farms is polluting lakes and causing large algae blooms that are harming fish populations, what is that phosphate doing to your pool?

What IT Does In Swimming Pools

In short, it causes algae growth and green pools. It causes a lot of headaches, even for seasoned pool professionals.

Most of the time a green pool can be combatted with high levels of chlorine (shocking a pool), cleaning the filter, and in some cases the use of an algaecide to kill the algae. We actually talk about that in detail here in our guide to removing yellow and mustard algae. This works really well, but phosphate is resistant to these measures. Even if you are able to clear the pool up the phosphate will accelerate any new algae growth, putting you right back to square one.

So if your pool is resisting and constantly turning green no matter what you throw at it, you likely have a phosphate issue. The first step to fighting phosphate, like anything pool related, is to test for what the levels are.

How To Treat Phosphate

You’re going to need a phosphate removal chemical. There are many different kinds out there but the one we use on our pools with high phosphate levels is this product.

  • Test phosphate level
  • Clean filter
  • Use phosphate remover
  • Run filter for 24-48 hours
  • Clean filter
  • Re-test and repeat until phosphate level = 0
  • Shock the pool

How To Test Phosphate Levels

General chemistry kits do not contain what you need to find your phosphate levels. Even the professional kits we use don’t come with a phosphate test mix. A search online for phosphate test kits will surely bring many different varieties but we tend to use these test kits.

Now unlike other chemicals that are measured in parts per million (ppm) phosphate is actually measured in parts per BILLION (ppb). It really does not take much phosphate at all to drastically affect your pool. In fact, as little as 300ppb is enough to render many treatments ineffective and anything over 500ppm is sure to keep your pool green.

How To prevent Phosphate From Returning

Understanding how phosphate gets into your pool is the first step to knowing how to prevent it. There’s a reason most public pools ask you to shower before entering. It’s not because of dirt, or anything like that its to try and remove things like hair conditioners, and accumulations of body oils that contain high levels of phosphate in them. Understanding your bather load and having practices that limit the phosphate they bring in with them to swim is a major key to lowering future issues.

Keeping your deck and screen enclosure clean will also help keep down phosphate levels. accumulations of pollen or even algae on these areas are not only unsightly, but they also bring all that phosphate into the pool when it rains. If you live in Central Florida as we do then you’ll know just how frequent that can be!

Testing every couple of months is a great idea if you receive a lot of swimmers or live in areas with significant runoff water from your yard and screen. This stuff accumulates over time and when you notice it, it needs to be handled immediately in order to get your pool turned around in days rather than weeks.


Are phosphate and nitrates the same?

They both cause large to grow faster in pools but they are indeed different chemicals compounds

Can chlorine remove high phosphates?

In high enough volumes it can help clear algae but it does not affect the phosphate level whatsoever

My pool is slimy, is that phosphate?

It may be. Phosphate can accelerate the growth of green algae to form slime-like deposits on the walls and floor of your pool

Can I Prevent Phosphate from getting into my pool?

Not Really. You can limit its introduction to the pool water but over time it will always accumulate.

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