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Fastest way to kill Black Spot Algae in your pool

In this article, we will show you how to spot black spot algae in your pool, discuss the best way to kill black spot algae and what type of preventative measures you should put into place

Black spot is a common algae problem found in fibreglass, pebble-crete, tiled and also marble-sheen pools. The problem with this algae strain specifically is its stubborn resistance to chlorine treatment. Its outer layers help to shield the algae from regular chlorine treatment, while its spores penetrate into pool surfaces.

Signs of Black Spot Algae

In order to differentiate black spot algae from pool marks and apply the correct treatment to stop it from taking over your pool. Below are the signs of black spot algae:

  • Black lumps in the shaded areas of your pool
  • Found more predominantly in the corners
  • Feels slimy to touch, similar to moss
  • Can be black, green or blue in colour
  • If left alone, you will notice the black spots spread and thicken

Can you swim in a pool with Black Spot Algae?

This is a great question and the answer would depend on your pool water quality and how bad the algae spread is. The algae itself does not pose harm to your health, however, the algae will deplete your pool chlorine, making it a perfect breeding ground for harmful bacteria such as E.coli to thrive.

How to get rid of Black Spot Algae

The below treatment consists of the combined use of designated algaecide along with high-strength granular chlorine. Together, they work to starve and clear the algae.

  1. Brush algae vigorously with stainless steel bristled brush to break down its outer membrane.
  2. Turn your pump off for 2 hours. After the water flow has still assess the algae and treat
  3. How strong is the algae’s presence?
    1. A few spots: Leave chlorine tablet to sit on algae overnight. If on walls, place a granular chlorine stocking or similar and position the chlorine over the affected area. Do this by tying off with string and lowering it to the desired height
    2. A large cluster: Funnel a large amount of granular chlorine ie 1-2 kg to the area and leave overnight
    3. Widespread: As above, using 10kg+ instead
    4. Entire pool (rare): You will have to take drastic measures of emptying the pool, blasting and acid washing the sides and floor.
  4. Leave pump off for another 24 hours then vacuum to waste
  5. Add algaecide after 3 days
  6. Bring a pool water sample into your local pool shop after 36 hours. Balance the water as directed by the pool technician
  7. Continue to brush the area twice a week for 4 weeks.


  • Black spot may only reappear if all steps including step 4 have not been carried out
  • Correct water balance must be maintained for best algae defence at levels of pH 7.2 - 7.6, Chlorine: 2-3ppm all year round

How to prevent an algae problem in your pool:

Save yourself time and money by preventing an algae outbreak. This can be done by taking the following measures:

  • Maintaining your pool equipment. Ensure there is a good flow, filtration and sanitation
  • Regularly check your pool pH, alkalinity and of course your chlorine levels.
  • Give your pool a regular brush and vacuum
  • Your pool filter should be running between 8-12 hours a day during the swimming season.
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