How to Get Rid Of Blackspot Pool Algae

Black algae is perhaps the hardest algae to get rid of! It has very deep roots that can grow into the side of plaster or concrete pools. It also has many layers that protect it from normal chlorine levels. You will usually find black algae clinging to your pool walls or steps in very shady spots. So in order to get rid of this algae, you must be aggressive!

 Here is a step by step guide to get rid of this slimy, stubborn creature.

Table of contents

1. Check the water chemistry in your pool

2. Brush the pool surface

3. Keep your chlorine levels nice and high

4. Brush your pool again and again

5. Vacuum any waste

6. Clean your filter


Warning: Be prepared to do a lot of brushing. We suggest buying a good quality brush for this procedure. 

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1. Check the water chemistry in your pool

Make sure your pH levels always sit between between 7.4 and 7.6 and your alkalinity levels are between 120 and 150 ppm (parts per million). But, if you're getting rid of blackspot algae, keep your pH lower at around 6.8 - 7 for 2-3 months. Algae loves high pH, so this will slowly kill it and its roots over time.

ALSO SEE: How to test and fix your pool water!

2. Brush the pool surface

I cannot stress this enough. This will be the key to getting rid of black algae. Brush hard, brush thorough, brush often. Black algae forms a membrane which needs to be broken for treatment to work.

Brush the walls and floors with a steel bristled brush or a stiff plastic bristled brush for plaster pools.

3. Keep your chlorine levels nice and high

You're going to mainly be focusing on adding hydrochloric acid to your pool to lower your pH, but that of course goes hand in hand with making sure that your pool is sanitised with chlorine to help kill off the algae.

4. Brush your pool again and again

Brush it at least 3-4 times a week concentrating on where you first spotted the algae. Remember, black algae has deep roots that can be embedded into your pool walls. It may look like it’s gone, but it’s probably still in there. Just keep brushing. Continue brushing for several days after you see it starting to dissipate. 

This is not a guarantee. Black algae is a bastard and could continue to infect your water. If it does, you know how to kill it, just be more aggressive next time. Perhaps an additional dose of acid and chlorine and constant brushing.

5. Vacuum any waste

To prevent any spores recirculate into the filtration vacuum to waste any debris on the floor of the pool.

6. Clean your filter

At the completion of your water treatment, thoroughly clean your filter. This includes manually washing your cartridge filter or back washing your sand filter.

(TIP: Soak your cartridge filter element in a tub of clean water and a cup of Nappy San overnight. This will loosen up all the solidified debris in the pleats, allowing for a much thorough clean with the hose.)


Keeping your pool properly balanced and sanitised is the key to keeping all forms of algae from growing. Make sure you

  • Keep your pH, alkalinity and chlorine levels in the correct range at all times.
  • Run your pump and filter for 8 to 12 hours a day all season long.
  • Keep your pool clean by regularly vacuuming and brushing.

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Post author:

Natalie Hintze

Head of eCommerce & Marketing at Mr Pool Man,
Co-Founder at Water TechniX


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