How to Kill Black Algae in Your Swimming Pool

Pools require maintenance, and one aspect of it is getting rid of algae. While you might believe that green algae are annoying – they truly are! – other types of algae, especially the black-colored one is even worse. Although it is extremely difficult to get rid of it, it isn’t impossible to do so, but it will require a lot of work from your part, as well as patience and devotion to remove it completely from your pool.

What is It?

Although part of the same family as other types of algae, the black one is similar in cellular composition to a bacteria rather than a plant. Once it has taken roots, it is extremely stubborn and will refuse to leave the area on its own. And to make it even more difficult, it harbors several protective layers which make it immune to natural amounts of chlorine. Thus, it requires a lot of work to get rid of it.

If its black color doesn’t bring a warning, you should be aware of its properties. In itself, it’s not dangerous to humans, as it attacks plants and microorganisms in the water. However, it releases a toxin which is a highly potent poison that can affect humans and pets alike. Swimming in a water body contaminated by the black alga is dangerous, as the toxin will affect you nonetheless. It is even more dangerous if you accidentally swallow the water containing it, as it can lead to serious internal problems.

Prevention Against It

The best offensive method is a strong defensive, as it’s easier to prevent its apparition rather than having to deal with it later when it’s already made itself comfortable in your pool. It can be transmitted in two ways: airborne and on clothes. Although there’s no way of stopping the airborne spores from entering the water, you can actively fight against the spores that reach the clothes.

  • If you’ve swum in other water bodies besides your pool – ocean, sea, river, pond, lake – make sure to thoroughly clean your swimwear when you return home. Don’t simply wash it as you’d do with your regular clothes, but scrub it up and use bleach or any other detergent that contains sodium hypochlorite. Extend the same care to any items you’ve brought into the water, as well.
  • Rinse before entering your pool to ensure you’re not taking any remaining spores into the water.
  • For the next few weeks shock the pool weekly and preserve the water balance as level as you can.
  • Clean all the pool equipment thoroughly

How to Identify It

  • Due to its nature, it looks like black mold, with blue-green tints
  • It doesn’t simply float on the water, it is kept in place by a black floating root
  • It won’t get off by a simple brushing
  • Though it might look similar to a stain, it can be scrapped whereas the stain cannot.
  • Tends to appear in rough areas, where it can sink its roots into the material. Its preferred textures are concrete, gunite, and plaster, due to their high porosity, and it is likely to avoid fiberglass and vinyl.

How to Fight It

Even though it is a highly stubborn microorganism, it also requires a lot of effort on your part if you wish to efficiently get rid of it. Therefore, in your fight against it, you’ll require the following utensils:

  • A means to test the water, either strips or liquid tests
  • Chemical-resistant gloves and goggles
  • A telescopic pole
  • An algae brush – preferably with stainless steel or nylon bristles
  • A handheld wired brush or pumice stone
  • Shock water treatment – enough for several rounds
  • Two new filter mediums – depending on what type of filer you own
Cleaning Steps
  1. Once the bacteria have reached the water, more likely than not, it has also infiltrated into the filter. So, before you start, change the filter medium with a new one.
  2. Check the water chemistry and bring it into balance. Since the black invader thrives on high pH, low chlorine and high alkalinity, leveling these three elements will start the fight
  3. Brush the area where the black spots are. But do it consistently, thoroughly and vigorously, as this will help get it off its roots.
  4. Swipe the telescopic brush with the handheld brush or pumice stone, equip the goggles and gloves, and go up-close. In this step you can also utilize the chlorine tablets together with the scrubbing medium, as it will force the sanitizer directly against the bacteria, lowering the latter’s defenses.
  5. Once the black dots have diminished, switch back to the telescoping brush and give it one more good scrub before advancing to the next step
  6. Shock time! Calculate the required amount of shock treatment for your pool capacity and multiply it three or four times. You might think this is a bit excessive, but the pathogen is immune to normal dosage, so you’ll have to add more if you want it gone
  7. Run the pump for an entire day and let the new filter remove as many spores as it can. During this time you can individually disinfect the brushes and, with clean accessories, brush it up once again.
  8. Once the 24 hours are up, stop the pump, clean the filter once again, and shock the pool one more time. Then turn on the filtration system.
  9. Test the water and bring its chemistry back into balance.
Keep in mind that shocking your pool should be done when the sun is no longer a problem. The best time to engage in this activity is at dusk or even during nighttime

Final Thoughts

Having to deal with black algae is not pleasant. It requires lots of patience, hard work, and determination to get rid of it. Thus, it is easier to prevent its apparition rather than removing it. If you keep the pool equipment and accessories clean and preserve the water’s balance, surely you won’t have to deal with it too soon.


  1. Jannet says:

    Will cleaning the pool regularly with an automatic pool robot help prevent the appearance of black algae?

    • Becky Eddinson says:

      Cleaning your pool on a weekly basis with a pool robot will help you remove spores and prevent algae from taking hold. Also, you can use a steel or a nylon brush to clean the walls, floors, and steps of the pool.

  2. Andrew says:

    Can I swim in the pool while it’s being treated for black algae?

    • Becky Eddinson says:

      It is not recommended to swim in a pool while it is being treated with various chemicals because they can be absorbed through your skin and can be extremely dangerous.

  3. Briggite says:

    How long after treatment with algaecide is it possible to swim in the pool?

    • Becky Eddinson says:

      After using the algaecide, you must check the chlorine level. When the chlorine level returns to normal, you can use again your pool.

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