How to Kill Black Algae in Your Pool

Written by Michael Dean
August 11, 2023

removing black algae from a swimming pool

Keeping your pool clean is one of the most important lessons I will preach. Failure to keep your swimming pool clean can cause all sorts of problems. Black algae is one of the worst contaminants you will come across.

Due to black algae’s appearance, it’s relatively easy to spot if it grows in your pool. Killing it isn’t always the most straightforward task, but it’s necessary since black algae can be nasty for your health. In this guide, I will tell you what black algae is, how to prevent it, and a step-by-step process for removing it from your pool. For more cleaning tips, read my guide on how to clean a pool.

Main Takeaways

  • Black algae is a cyanobacteria that is notoriously difficult to kill.
  • Black algae often appears as “black mold” on pool surfaces.
  • To kill black algae, you will need to brush the pool’s surfaces, scrub the black algae, and quadruple-shock the pool.
  • To prevent black algae infestations, you should shock your pool regularly, stay on top of pool chemical levels, and avoid swimming in the pool with dirty swimsuits.

What Is Black Algae?

Black algae is not true algae, but it is very similar. It is a single-celled organism called cyanobacteria that is generally blue-green. Like other aquatic plants, cyanobacteria contains chlorophyll, which gives it its green color. However, black algae has a water-soluble pigment that, when combined with the blue-green color, gives it its signature black appearance.

Instead of floating on the surface of your pool water, black algae prefers to root itself into porous surfaces, such as your pool’s plaster or concrete lining. It can often look like spots on your pool’s surface, like the image shown below.

Black algae is so tough to kill because it is encapsulated bacteria – it has a hard outer shell that allows it to survive various harsh conditions. It’s difficult to kill it with chemicals alone. Instead, it’s best to use a strong chemical sanitizer. A chemical sanitizer can break the shell, dislodge the algae from your pool walls, and eventually kill it.

Side note – if you’re fighting algae, check out my guide on how to quickly kill algae in your pool.

What Causes Black Algae?

Like other types of algae, black algae can enter your swimming pool on swimwear, from dogs jumping in the water or rainwater washing organic materials into the pool. Be sure to wash your swimsuits and take a shower before jumping in your pool, especially after swimming in natural bodies of water. Generally, if your pool is chemically balanced, black algae will not thrive or overtake your water. So you must maintain the chlorine and pH levels.

Is Black Algae Dangerous?

Cyanobacteria can create cyanotoxins, which can make people and pets very sick. Even simply swimming with black algae can be very dangerous, but ingesting water infected with black algae is a severe health hazard that can leave lasting damage.

How to Recognize Black Algae

Black algae doesn’t float on the top of your pool like some algae types. But thankfully, black algae is easy to spot. You should look for patches of “black mold” on the sides of your pool, under pool covers, and on any other porous toys or surfaces. However, remember that dirt and stains can often be mistaken for black algae.

Due to the encapsulated nature of black algae, it can survive in chlorinated conditions. If black algae is a persistent problem in your area, invest in a pool cleaner specifically designed to kill black algae. You’ll probably still need to pair this formula with some good scrubbing.

If you suspect that you have a patch of black algae in your pool, don’t risk your health by going in! Black algae usually only causes an upset stomach, but it can occasionally cause permanent liver damage or neurological disease when ingested.

How to Kill Black Algae: Step-By-Step

Killing black algae in your pool can take some time. You’ll likely need to disinfect every affected surface while scrubbing everything thoroughly to dislodge the algae and break its heads. While some people prefer to drain their pool to make this step more manageable, it’s not required. Here are the steps you can take to kill black algae before draining your pool.

  1. Clean or replace the filter
  2. Test the water
  3. Brush the pool
  4. Quadruple shock the pool
  5. Clean the filter
  6. Shock again if needed
  7. Test the pool water

I cover each of these steps in more detail below. But first, gather your supplies.

Supplies Checklist

  • Pool brush
  • Testing kit
  • Bucket
  • Shocking chemicals
  • Bleach-water solution

1. Clean or Replace the Filter

It’s a good idea to remove and sanitize the filter during this step. Either soak it in a weak bleach-water solution to clean it or replace it entirely if you have an incredibly persistent infestation. Soak any pool accessories also, like skimmer baskets and vacuums.

2. Test the Water

Test your pool water’s alkalinity, pH, and sanitizer. Know what the optimal levels are and meet those. Adjust the levels accordingly before moving on to the next steps. Getting your water levels right will help you decide how much pool shock to use in step 4.

3. Brush the Pool

By brushing the algae off of the surfaces of the pool, the algae will be released into the pool, where the shock will be able to kill the majority of it.

Make sure to get every possible crack to remove the algae from sticking onto anything. It’s a good idea to use a stronger, stainless steel brush for this instead of your everyday pool brush.

Focus especially on the areas of your pool where the algae didn’t come off. Use a stone, knife, or small wire brush to scrub these areas.

You can also try using a chlorine tablet, which will start to kill the bacteria while you scrub, offering double effectiveness.

After scrubbing your pool and the hard-to-reach places, brush your pool once more. If you have the time and energy, brush and scrub a third time as well. The more you brush, the better!

4. Quadruple Shock the Pool

Follow up your brushing with a powerful chlorine shock to your pool, and a big one at that – four times the usual dose recommended by the manufacturer of the pool shock you use. Use my pool volume calculator to figure out just exactly how much shock you need to use. Make sure to shock the pool at night, as the sun will render much of chlorine useless if you shock during the day.

Pro tip – place all your pool equipment and accessories in the shallow end of the pool so they can get sanitized during the shocking process.

Run your pool pump for at least 24 hours so the shock can be evenly distributed and adequately kill the bacteria.

After you’ve run the pump, brush the pool again.

5. Clean the Filter

After running the filter to disperse the shock, your previously cleaned filter will have collected more algae. The good news is, this time, the algae will be dying instead of living bacteria. Backwash or rinse your filter with a filter cleaner instead of replacing it this time.

6. Shock Again If Needed

If you see any remaining black algae in your pool after taking these steps, shock your pool again using a 2x dose of the manufacturer’s recommendations.

Run the pump for 24 hours, or 8 to 12 hours at minimum, and brush the pool again. I can’t stress the importance of brushing your pool. You don’t want to risk the black algae coming back if there is any left!

7. Test the Pool Water

Once you take all the steps, test the pool water and ensure the chemical levels are accurate. Your pool should not have any more black algae, but keep an eye out for any missed or returned algae. If even the smallest amount is left behind, it will start to reproduce and grow back. Give your pool a standard shock after brushing any missed algae off.

It’s a good idea to shock your pool regularly to keep the black algae at bay. Some pool owners do this as often as once per week with great success. However, recoating or replastering your pool’s surface is a nearly foolproof solution to an unfixable black algae problem if all else fails.

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How to Prevent Black Algae Infestations

As you may have realized, the most efficient way to take care of a black algae infestation is to prevent it from ever happening in the first place! You can’t always stop the spread of the black algae, but here are my pro tips for preventing it from showing up:

Don’t swim in your pool with dirty swimsuits. Black algae spores are plentiful in nature, and they often hitch a ride on swimsuits and find their way into your pool. Wash your swimwear thoroughly before jumping into the pool.

Think twice before letting your dog jump in. Your pet may love your pool just as much as you do. But animals can carry all sorts of contaminants on their bodies, including black algae. So unless your dog has been freshly bathed, I would not recommend it from a hygiene perspective.

Stay on top of your chemical levels. If you dutifully maintain your pool’s chemistry, you are much less likely to have a black algae infestation. The more regulated your pool’s aquatic environment is, the less likely bacteria and algae will thrive there. A robust and functional filter that you run often can reduce risks too.

Shock your pool regularly. Shocking regularly is a great strategy to keep your pool healthy. While black algae can tolerate intense chlorine levels, a highly-chlorinated pool is still better for discouraging infestations in the first place.

Questions? Let me know, and I’ll be happy to help.

Frequently Asked Questions

Should I drain my pool to get rid of black algae?

While simply draining your pool won’t effectively get rid of the black algae in your pool, you can drain your pool to get rid of it by following some additional steps. If you want to drain your pool to get rid of black algae, you must acid wash the surfaces. An acid wash can be an effective black algae treatment. With that said, however, if black algae has taken root in your filtration system as well, following my step-by-step guide above would be the best solution.

What’s the best black algae killer for pools?

The best algae killer for pools is chlorine. However, if you’re looking for something else that might help your chlorine do its best in killing the black algae in your pool, try supplementing the shock or chlorine with an algaecide that works against pesky black algae.

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