It’s bad enough when your fibreglass pool is infected with green algae. But usually, dealing with green algae isn’t all that hard. In case you find black algae in your pool, on the other hand, then you’ll have to work harder to clean your pool. When there are black algae in the pool, no one should be allowed to swim in it until the pool has been cleaned.

How is Black Algae Formed?

If you find black algae in your pool, then you may be wondering how it got there. Someone, who swam in a body of natural water such as a pond, a river, a lake or even the ocean could be responsible. They probably didn’t clean their swimwear after and may have used the same swimwear in your pool.

It isn’t just swimwear that’s exposed to natural water bodies, but anything from goggles to pool floats that have been exposed could lead to black algae forming in your pool. While it’s less common, sometimes, black algae can also enter your fibreglass pool through airborne spores. Before you can get rid of the black algae that are in your pool, you’ll first need to understand what it is.

What Is Black Algae?

Algae is actually an aquatic plant. It belongs to a group that includes everything from organisms that are single-celled, to even seaweed. This is why when people picture algae, they usually think of something that is green.

In the same way most plants contain chlorophyll, so do green algae. Chlorophyll is the pigment that provides green algae with its signature green colour. Chlorophyll is also what allows plants to perform the essential function called photosynthesis. This is the process that plants use in order to synthesize food from CO2 as well as water.

Black algae on the other hand are actually bacteria. To be more specific, this is a kind of cyanobacteria. Another name for this is blue-green algae, thus ‘cyan’ is added to its name. Now that you know that black algae are actually cyanobacteria that are still blue-green coloured, you may be wondering why it’s called black algae. This is because cyanobacteria contain chlorophyll. This chlorophyll creates the blue-green colour. But other pigments that are soluble in water could also be present. When these pigments combine with the pigments coloured blue-green, then the bacteria appear to look black.

How Do You Kill Black Algae?

It’s the fact that black algae are bacteria, that makes it so hard to kill. However, this doesn’t mean that it’s impossible to kill. You’ll need to put in more effort and be more persistent, and then you’ll be able to get rid of the black algae in your fibreglass pool.

Here are the supplies that you’ll need to get rid of black algae:

  • A backwashing hose
  • A filter cleaner
  • DE powder ( if necessary)
  • Filter sand (if necessary)
  • Filter cartridge replacement (if necessary)
  • Liquid test kit
  • Test strip
  • An algae brush
  • Gloves that are resistant to chemicals
  • Safety goggles
  • Chlorine tablet holder
  • Chlorine tablets
  • A telescoping pole
  • The shock of calcium hypochlorite

So how do you actually get rid of black algae from your pool?

1. Cleaning Your Filter

When there are black algae in your fibreglass pool, that black algae will also enter your pool filter. Should there just be a small number of algae in your pool, then all you’ll need to do is backwash your pool filter. If you have a cartridge filter, then you’ll need to rinse it.

Should you have extensive algae, then you’ll need to use a filter cleaner. When your pool is overridden with black algae, consider replacing the filter cartridge or the medium. You’ll be able to start afresh while enduring that there’s not even a speck of black algae left in your pool filter. For more specialised knowledge on this issue, reach out to the Pool Builders Sunshine Coast.

2. Testing And Balancing The Water

Ensuring that the levels of your pool water are where they should be, will help you determine how much pool shock you need to use. To do this, you can make use of either a liquid test kit or test strips. For now, you’ll need to focus on your pool’s alkalinity, as well as its pH.

The alkalinity of your pool should be between 100 to 150 ppm. The ideal level is 125 ppm. The ideal pH for your pool should be between 7.4 and 7.6. The amount of chlorine in your pool should be between 1 and 3 ppm.

3. Brushing Your Pool

You’ll need to brush your pool thoroughly. A special brush made of stainless steel bristles should be used for this purpose. You’ll be able to get into any cracks as well as crevices in order to dig the black algae out. When you brush the black algae, it becomes loose and enters the water. There, the pool shock can kill it.

After you’re done brushing, use a putty knife or even a pumice stone, to scrub black algae off the surface of the pool. You can also use chlorine tablets at this time, which will start killing the bacteria.

4. Shocking Your Pool

It isn’t easy to kill black algae. First, determine how much shock you would normally need to add to your pool. Then, add four times that amount of shock. You can also use calcium hypochlorite shock as well, as it is effective at killing black algae.

5. Run Your Pool Pump

In order to ensure that the shock is dispersed, you’ll need to run your pool pump for 24 hours. Don’t worry about the black algae entering your filter at this point. This is because the shock will already be killing the bacteria by this point. You’ll also need to clean your pool filter again and brush it as well.

If the black algae still aren’t gone by this point, then you’ll need to add shock to your fibreglass pool again.


If you have black algae in your pool, then you’ll need to work hard in order to get rid of it. From adding shock to your pool to cleaning the pool filter, this guide details how you can get rid of black algae from your pool. Follow the instructions in this guide to remove black algae from your pool effectively.

Emma Chen

Emma Chen holds a degree in Public Health from the University of Washington and has dedicated 13 years to promoting healthy and sustainable cleaning practices. Since joining us as a freelancer in 2020, Emma has shared her expertise in non-toxic cleaning solutions, indoor air quality, and allergen reduction. Her experience includes working in community health programs and as a health educator, which shows in her writing. Emma is a yoga instructor in her free time and participates in community clean-up drives.

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