Pool Phosphates: What They Are and How to Lower Them

You have kept the chlorine steady and the pH balanced, yet green or cloudy water keeps returning. If you have run out of explanations, phosphates may be the missing piece. In this guide we explain what pool phosphates are, whether they really matter, and how to bring them under control.

At HiLife Pools we talk to pool owners about this regularly, so here is the plain version, without the sales pitch.

What are pool phosphates?

Phosphates are compounds containing phosphorus, which is a nutrient for plants and algae. They are everywhere: in fertiliser, leaves, rain, tap water, suncream and some pool chemicals. A little will always find its way into your water.

Algae needs phosphates to grow, much like a lawn needs feed. With plenty of phosphates available, any algae that appears has a ready source of food.

Do phosphates really cause algae?

Phosphates do not cause algae on their own. Algae spores arrive on the wind and in rain, and they only take hold when your chlorine is too low or the water is out of balance. Phosphates simply make the problem easier for algae and harder for you to clear. If your sanitiser is strong, a moderate phosphate level is rarely an issue. If your pool has stubborn algae, it is worth testing. Our guides on pool algae and mustard algae in pool cover the main types.

Where do phosphates come from?

• Leaves, grass clippings and garden fertiliser blowing or washing in.
• Rain and tap water, which vary depending on where you live.
• Suncream, body oils and cosmetics from swimmers.
• Some pool chemicals and cleaners, which can add small amounts.

How to test for phosphates

A phosphate test kit is inexpensive and available from most pool shops. The result is usually shown in parts per billion (ppb). Many pool professionals aim to keep levels below about 300 to 500 ppb, although there is no single official limit. Test at the start of the season, and again if you see unexplained algae.

How to lower phosphates in a pool, step by step

1. Clean the pool thoroughly. Remove leaves, brush surfaces, empty the skimmer baskets and clean the filter. Our guide on cleaning a pool filter will help.
2. Get the chlorine and pH right first. A balanced pool handles most phosphate problems on its own. Our guide to shocking a pool explains what to do if the chlorine has dropped.
3. Test the phosphates. Record the reading so that you can see whether treatment works.
4. Use a phosphate remover if needed. These products, usually lanthanum based, bind with the phosphates so that the filter can trap them. Follow the label closely, because overdosing can cloud the water.
5. Run the filter and backwash or clean it. Keep the pump running for 24 hours and then clean or backwash the filter to remove the trapped material.
6. Retest. Check the reading a day or two later, and repeat only if necessary.

Is draining the pool an option?

Partial dilution with fresh water lowers phosphates a little, but if your tap water contains phosphates, the benefit is limited. For most pools, a remover product and good housekeeping are more effective.

How to stop phosphates building up

• Keep trees and plant beds tidy, and skim leaves quickly.
• Ask swimmers to shower before getting in, to cut down on suncream and oils.
• Use a cover when the pool is not in use.
• Follow a regular routine such as our pool maintenance checklist.

Frequently asked questions

What is a normal phosphate level in a pool?

There is no official limit, but many professionals aim to stay below 300 to 500 ppb.

Are phosphates harmful to swimmers?

No. They are not a health risk, but they feed algae, which makes the water harder to keep clear.

Do I need a phosphate remover every year?

Not necessarily. If your chlorine is well managed and the pool stays clear, you may never need one.

Can phosphates cause cloudy water?

Indirectly. They help algae grow, and algae is a common cause of cloudy water. Overdosing a remover can also cloud the water for a day or two.

Still battling green water?

If algae keeps coming back, or you would like help working out why, get in touch with the HiLife team. We are always glad to help.