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How to clean and fix a green swimming pool

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Does your pool water look like this?

Summer in South Africa with our hot and windy days days can cause havoc with a swimming pool. A pool that is normally a bright blue colour can within days turn into a green slimy monster that even frogs wont find their way in. This is what has recently happened to me. I kept adding chlorine and then acid and the pool just got worse and worse. So bad in fact that when you walked past it your face took on green glow from the reflected light.

What to do to fix something like this?

It's not just a case of adding chlorine and acid which you can do till you are blue in the face because if your pH is not right your water will stay green.

Here are the steps you need to follow to clean out the green algae in your swimming pool

Get yourself a test kit and test for two things to start with.

pH Test

What is pH and why do we have to measure it?

The reason we test the pH of water is to determine whether it is pure or not . Looking at a green pool you can tell that it is not pure but you still don't know what is wrong. The pH test tells you whether the water is too acidic or too alkaline by the readings you get when you compare the colour of the test to the colour chart you get with your pool test kit. With a pH test if the reading is below 7.00 then your acidity in the pool is too high. In my case it was off the charts on the colour charts and showed up as the colour of urine. If the test reading comes in at 7.6 or above for a marbellite pool or 7.2 and above for a glass fibre pool then the water has a high alkalinity.

 test kit

Test Kit

Alkalinity Test

Assuming that your pH test reading is below 7.00 the next test you do is the Total Alkalinity test.

Using the larger tube of your test kit (see picture above) fill it to the level marked alkalinity and follow the instructions as to how to do the test in your test kit booklet.

After adding one drop of solution 4 and one drop of solution 5 the water in the test tube will turn a violet colour.

Now start adding solution 3 a drop at at time counting the drops as you go along and when the water permanently changes to yellow multiply the number of drops added by 10 to get the reading of the total alkalinity of your pool. In my case the water changed colour with the first drop which meant that I actually had a very acidic pool and no alkaline in my water at all.

As the ideal reading should be in the region of 80 to a 120ppm ( parts per million) I needed to add a kilo of alkalinity up (bi carbonate of soda for a glass fibre pool or soda ash for a marbellite pool) for each 10 parts of alkalinity the pool needed. My pool was so bad that it took 20 kilos of alkalinity up to get to the ideal reading of 100ppm.

Once you have the alkalinity sorted out check the pH again and if this is in the range 7.2 to 7.6 then the pH of your water is fine.

 test kit chemicals

Chemicals in your test kit

If your pool readings go to the other extreme and the pH comes in at 7.6 or higher then your pool's alkalinity is too high and needs to be adjusted. The test you need to do to determine what you should do next is called the Acid Demand Test.

Using the same water you used for the pH test start adding drops of solution 3 counting them as you go. After each drop the colour of the water will change slightly until it reaches the orange colour labelled 7.2 on the test kit. Your pH is now at its required level in the test tube. It is now time to take action. Firstly you should know the size of your pool in litres and if it is around say 50000 litres and you counted 5 drops then referring to the acid demand chart in your test kit you should add 1875 millilitres of acid either in liquid or dry acid form. Allow the acid to mix through the pool for a few hours and then test the pH again. If the pH is in the correct range then go to the next step.

Ah gee! You've done all this measuring and testing and adding of chemicals and your pool is still green. The problem is that you now need to remove all the dead algae and dirt in the water from your green swimming pool.

Removal of dead algae from your green swimming pool ( Do this step in the evening).

For this step you need to remove your automatic pool cleaner from the pool and switch off your pump once you have added the chemicals you need for this step.

The best chemical I have found to clean the pool water is Alum although there are other flocculants that also do the job.

Mix the alum in water in a bucket and pour it around the sides of your pool and throw some into the centre of the pool to ensure that the alum mixes with all the water in the pool. You can use the pump to assist in the mixing but as soon as you are happy that the alum is in solution switch off the pump and keep it off until the following morning.

When you get up take a look at your pool and you will find that most of the sediments (green algae) have sunk to the floor of the pool and the water is clear.

Pump out the sediment.

To do this step you need to use a hand held pool vacuum broom which must be connected to your pump. Set the pool filter to waste and then slowly vacuum the sediment off the bottom with the broom. It is necessary that you do this very slowly as sweeping quickly will cause the sediment to rise and go back into suspension.

Once you have swept the whole of the pool removing all the sediment at the bottom of the pool switch off the pump disconnect the broom and put the automatic cleaner back into the pool. Backwash your filter and then set your automatic cleaner to its normal cycle.

After pumping the sediment out of your pool it will be necessary to refill your pool. Once you have done that check the pH again and adjust your chemicals accordingly.

After going through the whole procedure set out above my pool although blue in colour was still dusty and slightly murky, it did not sparkle like I knew it could. There was still something wrong with the water. After a lot of head scratching I finally decided to check the sand in the pool's sand filter. It was a sticky mess. I removed the old sand and replaced it with new and today the pool sparkles like it should and the automatic cleaner runs around the pool like something is chasing it.

 clean pool

What your pool water should look like after the above process

Have you done all of the above and your pool is still green. Are you frustrated and want to fill the pool with sand. Well join the club, so am I. There is another problem in your pool that normal testing does not reveal and if this chemical goes out of balance you have endless problems fixing it.


The job of stabiliser in your pool is to keep the chlorine levels up and to stop the sun drawing it out of the pool. Stabiliser is not like other chemicals that disappear over time. It stays in the pool and builds and eventually when you start having problems you have no idea what has caused them as acid levels, chlorine levels and ph are all perfect and your pool is green.

You have what is known as a chlorine lock where the stabiliser grabs any available chlorine in the pool and stops it from doing what it is supposed to and that is to kill off the algae in the pool.

The first thing to do when you find this happening is the get some pool testing dipsticks to test your pool with. These dipsticks take four readings and include the stabiliser levels in your pool. A quick dip of the dipstick into your pool will reveal the stabiliser problem you have and you will be able to start fixing it.

If the stabilser reading is very high ie over 150ppm you have quite a large problem on your hands. You have to reduce the stabiliser levels in the pool and the only way to do that is to dilute the stabiliser. This means that you have to pump out water and put in fresh water until you can reduce the levels of the stabiliser to the required levels. ie less than 60ppm

In some instances it might be necessary to empty your pool completely and start over. When doing this remember that emptying your pool might result in it popping out of the ground because of the ground water levels. It's best not to empty the pool completely but to reduce levels and fill until such time as the stabiliser has been diluted to within the required or recommended levels.

You may ask. "How did I get so much stabiliser in my pool as I have not consciously been adding it?" The answer is that if you are adding stabilised chlorine or chlorine pill floaters you are adding stabiliser each and every time you put a new pill into the pool. As the levels of stabiliser build up your pool starts to give you problems.

Beware of the floating pills and 4 in 1 packages as they are the cause of your problems. Do not put them in your pool if your stabiliser levels are high. Use granular chlorine instead or otherwise get yourself a salt water chlorinator which is most probably the best way to go.

Good Luck!

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Fix green swimming pool

Page Updated 14.3.2017