How to Identify and Treat Black Algae in Pool

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What is Black Pool Algae?

Black pool algae (cyanobacteria) or blue-green algae isn’t really an algae species. Instead, it is a type of bacteria that grows in dense colonies. This bacterium roots itself against the pool walls and floor surfaces. The algae are relatively easy to remove from smooth pool surfaces like fiberglass pools but can be very hard to scrub or scrape from porous or rough surfaces like concrete pools or gunite pools. 

Black algae in pools are very easy to identify because it looks and acts a lot different from green pool algae. This type of algae will first start as small pinhead-sized dots on your pool walls and floor. Over time, these dots will multiply and grow in size. 

Compared to algae species, this bacteria is very hard to clean because the species is resilient to pool algaecides thanks to a layer of slime it creates to protect the organisms. 

What Causes Black Algae in Pools?

Black algae usually aren’t introduced into pools through your local water supply or through rainwater. Instead, it is carried into your pool by people. When someone swims in a natural body of water like a pond, river, lake, or pool with black algae, the spores cling to your swimwear. These spores are then carried into your pool when someone swim’s in your pool when wearing the same swimwear. 

Black algae can be carried into pools through airborne spores but this is a rare occurrence. You can also get black algae when floodwaters carry these spores into your swimming pool.

Fungal spores are usually killed by recommended pH and chlorine levels.  But if the levels are not strong enough to kill the fungi, they will quickly deposit on your pool surfaces and grow. As it grows, it will become more resilient against algaecides and pool shock treatment. 

How to Identify Black Algae in Pools?

Black algae are pretty easy to identify because it looks quite different from normal algae. Unlike other algae varieties, this type won’t discolor your pool water. The water can still be crisp clean despite being contaminated with this type of bacterium. To identify black algae, you can look for the following signs;

  • Black or blue-green spots with raised heads on pool walls and floor
  • Algae grow in rough areas of the pool or on pool plaster
  • These growths do not brush off the surface easily
  • Black algae can grow in pools with proper filtration and sanitation systems
  • The black dots multiply and grow in size
  • The black spots resurface after scraping them off because this alga has deep roots
  • The growths usually create a slimy layer over the spots


Can You Swim in a Pool With Black Algae?

Black algaecides usually aren’t dangerous to human health. The bacteria will make your pool smell musky and can alter the taste of pool water. It can also make your pool look unsightly with black fungal spots all over the surfaces.

Cyanobacteria or black algae can sometimes become dangerous. When the bacteria bloom it can produce cyanotoxins. These toxins are some of the most powerful natural poisons in the world. When animals or people consume these toxins, they can become ill. Cyanotoxins have even been known to result in livestock deaths. 

Drinking cyanotoxins can cause many health effects like the following:

  • Nausea
  • Stomach cramps
  • Liver damage
  • Headaches
  • Neurological symptoms like muscle weakness
  • Vomiting
  • Diarrhea

Swimming in water with cyanotoxins can also irritate your skin, cause skin rashes, and can make your eyes red and sore.

You can tell when cyanobacteria are blooming by the look of your water body. If you notice foam, scum, or mats on the swimming pool surface that is bright green, blue, brown, or red then the black algae managed to bloom and the water is likely extremely toxic. 

It isn’t necessary to drain your pool water if you have black algae. There are treatments that can clear up these growths and restore your water quality. But if the algae do manage to bloom, it is best to drain your pool water and give your pool a proper deep clean before adding fresh water. 

How to Treat Black Algae in Pools?

This type of bacteria is a bit more challenging to clean from your pool and it might take a while before everything will be cleared up. Here is a quick look at the best steps and remedies for treating and getting rid of this type of algae. 

Scrub your pool surfaces

Black algae are resilient against algaecides because it creates a protective layer on top of the growth that shields it from pool treatments. Before you can start treating your pool water, you will need to scrape that slimy layer from the black spots. Grab a pool brush and scrub the entire pool surface and floor area thoroughly.

Scrub pool accessories

Black algae can cling to just about any surface or pool equipment. You should also scrub down and properly clean pool accessories and pool toys like flotation devices, pool noodles, balls, steps or ladders, and anything else that might be used in or around the pool.

Wash swimwear

All swimwear items should be properly washed in a washing machine with a strong detergent to kill any fungal spores that might have clung to your swimwear. 

Apply pool treatments

Use chlorine tablets or granular chlorine and a pool test kit to restore your pool pH levels to 7.4 – 7.6. You should also test and create an alkalinity level of 120 – 150 ppm. These levels are ideal for killing and eliminating microscopic fungal spores that came loose from brushing your pool.

You can also use your chlorine tablet to rub directly against the surfaces that are affected by black algae growth. This method will kill the roots and will keep them from growing back. 

Run the pool filter

Once you applied the pool treatments and scraped off all the surfaces, there will be plenty of dead algae cells and particles in the water. You should now run the pool filtration system and try to circulate the water as much as possible to filter out these dead deposits. 

Remember to clean or flush your pool filter regularly as you run the filter so it won’t become clogged up.

Repeat and maintain

Keep your pool pH chemical levels at an adequate level and keep scrubbing at black spots as they appear on the pool walls. Run your pool filter every day until all traces of black algae spots have completely disappeared. 

To prevent future breakouts, you should keep swimwear nice and clean and maintain proper pool pH and alkaline levels. 

Final Thoughts

Treating black algae in pool water can be challenging but with the right product and care routine, you can clear up these black growths and restore your water quality. If you notice other types of algae like green algae or mustard algae along with black algae in your pool then you can check out some of the other guides we have on AlgalWeb. On our site, we offer the best tips and hints for treating and maintaining any type of water body.

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How to Identify and Treat Black Algae in Pool

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