My Pool Is Green but Chlorine Is High. Why and How to Fix?




So you have a pool and you keep it nice and sparkling by adding some chlorine tablets each week. Then one day your pool suddenly turns green so you add more chlorine but it stays green. You test the chlorine level and it is high so why is it still green?

A pool can be green but have high chlorine levels when the chemicals are unbalanced. Too much combined chlorine, high pH levels and also high cyanuric acid (CYA) levels can cause the chlorine to be ineffective with both situations potentially causing algae to grow.

These are just the basics of why your pool is green but chlorine is high. Understanding how to fix this problem is important. There are several facets to pool care that can cause a green pool, learning which will help you create an ever-sparkling pool.

My Pool Is Green but Chlorine Is High
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My pool has plenty of chlorine so why is my pool green?

Most pool owners know that chlorine kills algae so it doesn’t make sense to them that there is algae in pool, but chlorine is high – if there is plenty of chlorine why is the pool still green?

Although this may be true there are a number of reasons why the chlorine that is in the pool may not be able to work.

1. Little or no free chlorine

To understand this, we need to explain the three “types” of chlorine in your pool:

  • free chlorine
  • combined chlorine (chloramines)
  • total chlorine.

What is free chlorine?

Free chlorine is the amount of chlorine that hasn’t been used up in effect so is still available to kill algae and deal with other contaminants.

What is combined chlorine (chloramines)?

Free chlorine reacts with other things in the water, killing unwanted algae and other contaminants. When it does so it reacts with them to form combined chlorine, also known as chloramines. Once this happens (sometimes called “chlorine lock”) it is effectively used up and no longer available to keep the pool clean.

What is total chlorine?

Total chlorine is, as the name suggests, the total amount of chlorine in the pool. This includes both the free chlorine and the combined chlorine.

When you measure chlorine, you measure the total amount of chlorine in the pool, which means that there might be more combined chlorine than free chlorine despite high total chlorine readings.

I suggest that you use a test system that measures the free chlorine as well as the total chlorine so you will then know what the actual situation is with regard to chlorine in your pool.

The JNW Direct Pool and Spa Test Strips give you readings for both total chlorine and free chlorine and also total hardness, pH, CTA (stabilizer), total alkalinity and even bromine if you use that.

Pool and Spa Test Strips - Quick and Accurate Pool Test Strips - 7-1 Pool Test Kit - 100 Bromine, pH, Hardness and Chlorine Test Strips - with E-Book - 100 Water Test Strips - JNW Direct

2. Cyanuric acid (CYA)

What is CYA?

CYA is short for Cyanuric acid. It is used as a chlorine stabilizer in pools to prevent too much chlorine from being lost due to the action of the sun. If you use chlorine tablets or granules then it is likely that they include a certain amount of cyanuric acid too.

If you see trichloroisocyanurate, potassium dichloroisocyanurate or sodium dichloroisocyanurate on the labels of your chlorine then it contains stabilizer.

Over time, it is possible for the CYA levels to increase to levels that may actually start to reduce the effectiveness of the chlorine. If it reaches levels above 50 ppm then algae can start to grow and the pool turns green.

If you are unsure what to do with CYA then read How to add stabilizer to pool

How to lower cyanuric acid in pool

The most common method of reducing the cyanuric acid levels in a pool is to partially drain it and then refill it to dilute the CYA in the water. If your levels are very high, over 100ppm for example, then you may need to totally drain the pool.

There are CYA reducer products on the market such as the Bio-Active Stabilizer reducer which will bring down your CYA levels if you do not want to partially drain and refill your pool.

3. Incorrect pH

The acid levels in your pool affect how chlorine will react. It may well be that your pool is green but chlorine is high pH is low too.

Too little and the chlorine is eaten up by sunlight. Too much acid and chlorine is blocked from working properly. The acid levels will not kill bacteria or algae in your pool, but they will determine how effective the chlorine will be.

The pH measurement determines how acidic or basic the water is in your pool. Areas with high rainfall will have more acidic water, while people who use well water may have more basic water. Both of these levels can damage your skin but will also determine how effective chlorine is in dealing with things such as algae.

A good range for pool water is to have a pH level between 7.2 and 7.8. This means that not only is it comfortable to swim in, but more importantly, chlorine can work properly.

Very basically, if the pH is lower than 7.2 (so it is acidic) you will need to consider adding an alkaline such as pH+, baking soda etc to raise the pH. If the pH is higher than 7.8 (so it is alkaline) then you need to consider adding an acid such as muriatic acid to lower the pH.

4. High phosphate

Having high phosphate levels can also cause algae to thrive (as phosphate is basically plant fertilizer and algae is basically a plant).

However, this isn’t generally a problem for many pool owners, particularly those that live in towns and cities and away from trees. It can build up by high levels of leaves and other organic material being blown into the pool and also in rural areas if fertilizer from surrounding fields runs off into your pool during rain.

To prevent a phosphate build-up in the future try to remove leaves and other organic matter promptly before they have a chance to break down.

5. Is your filter running for long enough?

If your pool pump is not running for long enough each day then it is possible that the water is not being filtered sufficiently enough allowing the water to become green.

You have to let the pool filter run for long enough each day to at least filter the total volume of the pool once. This often works out at around eight hours.

To work out how long yours should run for read my article – How long should you run a pool pump

What to test for when your pool is green but chlorine is high

Test the chlorine

Knowing how much chlorine you have in the pool is relatively easy to test using a good test kit or strip. Make sure that your test strips not only check the total chlorine but also the free chlorine.

The ideal level for free chlorine is 2-4 ppm and combined chlorine should be less than 0.5 ppm. If your readings are in this range then you need to look further by checking the pH levels.

If the combined chlorine is high but the free chlorine is low then that is the most likely cause of your problem.

algae in pool, but chlorine is high

Test the pH

If you have thrown a lot of chlorine in the pool, but it stays green, you need to test the water for pH levels. This is often the case after a lot of rainfall as the rains increase the acidity levels in a pool while diluting the chlorine.

The most likely cause of a pool having high chlorine but being green is because the pH levels are too low, causing the chlorine to be ineffective. High acidity (low pH) in the water does protect the chlorine from being used up by the sun but stops it from reacting with algae and other things.

The pH should be within the range 7.2 to 7.8. If it is below 7.2 then you should add some alkaline to bring the level into this range. Read my post How to raise the pH in a pool for instructions on doing this.

If the pH is within the range then you should test for phosphates.

Recommended Test Strips

AquaChek 7-Way Pool and Spa Test Strips - Silver Pool Test Strips For pH, Total Chlorine, Free Chlorine, Bromine, Alkalinity, Total Hardness, and Cyanuric Acid - Water Quality Testing Kit (100 Strips)

Aquachek 7-way test strips

These are the test strips I use, that can tell you the total chlorine/bromine, free chlorine, pH, total alkalinity, CYA and hardness levels.

Test for phosphates

Most home pool water testers will not show phosphate levels so you may need to take a water sample to your local pool store for them to analyze.

You can buy a phosphate test kit – the Aquacheck Phosphate Test Kit is quite inexpensive.

You can buy chemicals to reduce the amount of phosphate in your pool if these are shown to be high. One of the most popular on Amazon is the Clorox Pool&Spa 55232CLX Phosphate Remover. Make sure you follow the manufacturer’s instructions.

How to fix a green pool with high chlorine?

Once you have worked out why the chlorine is high but not working and then used the methods above to correct it, you can move on to actually getting rid of the algae.

Shock the pool

Shocking the pool does two things to kill the algae and bacteria in the pool, the first is adding more chlorine, and the second is releasing the combined chlorine in the water. We recommend always checking the possibility of adding a quick shock treatment when struggling with a green pool.

All shock treatments will have measurements on the back that you need to follow, the chief of which is what levels the pH in your pool must be. When you are about to shock your pool, you need to make sure that all the other chemical levels are perfect for getting the best possible results.

Once the algae are dead it will be lying on the bottom of the pool. The only easy way to get rid of it easily is to vacuum the pool to waste as all the algae will be expelled from the pool.

Drain and add new water

Draining your pool, at least partially, and then topping up the water will help to reduce the amount of combined chlorine, phosphates and the pH of the water. This will have the effect of almost starting again from scratch, enabling you to remove algae and adjust the chemicals properly.

Do not use algaecide

Algaecide is not really used to kill the algae in a green pool as it is actually used more as a preventative measure to prevent it from growing in the first place. However many people just add lots of algaecide and then wonder why it hasn’t worked.

Shock chlorine will actually also help to prevent the algaecide from doing anything too.

Pool Care Handbook and Video Course

When I bought my house with a swimming pool, I knew absolutely nothing about pool care. I just winged it for a while, making many mistakes along the way.

Fortunately, I was recommended Swim University’s Pool Care Handbook and Video Course. I bought it and it was an absolute game-changer.

It was the best money I spent that year. I learned everything from basic cleaning to advanced troubleshooting. Swim University offers a no-quibble refund policy too so what do you have to lose?


It may be that you still have algae in pool but chlorine is high. That could be because your pool will be green even with high chlorine when the chlorine is prevented from working. You need to work out why chlorine is not available to kill the algae and then take steps to correct it.


Why is my pool still green after adding chlorine?

Your pool is probably still green even after adding chlorine because you have too little free chlorine, the pH of your water is incorrect or you could have high phosphate levels.

Why does my pool turn green when I add chlorine?

This usually happens when the pH in the pool is incorrect. It should be between 7.2 and 7.8

Is it safe to swim in a green pool?

Although algae grow in natural ponds and lakes, and people swim in them, it is probably not such a good idea to swim in a green pool. While many people will be safe, it may affect some. So if in doubt don’t swim but instead get rid of the algae using pool shock to kill it.

Can you swim in a pool with high phosphates?

It is safe to swim in a pool with quite high phosphate levels. If the levels are over 1000ppb then it can become dangerous.

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