How Do I Get Rid of Black Algae in My Pool?

discolored pool

Algae are single-celled plants that thrive in sunny, warm, nutrient-rich locations, making your swimming pool an ideal place to grow and spread. There are three main types of pool algae: green, yellow (or mustard), and black (or blue-green).

Each type presents its own challenges when attempting to eliminate it from your pool; however, some are more persistent than others.

What is Black Algae?

Black algae is a strain of algae that tends to grow on porous surfaces like the walls and flooring of a pool. This alga is dark in color, sometimes referred to as blue-green algae, and encourages the growth of bacteria.

When it comes to algae, black algae in your pool is the worst of the worst.

What Causes Black Algae in Swimming Pools?

Like all algae, black algae flourish in water, where they can photosynthesize and easily multiply. Growth is particularly common in pools with pH imbalances, low chlorine levels, and poor water circulation and filtration.

If your water levels aren’t falling within the recommended ranges, or your pool has been contaminated by another object — such as a toy or bathing suit that was recently in a natural body of water — black algae may begin to settle in the cracks, corners, and hard-to-reach places in your pool. Eventually, it will penetrate through concrete, exposed aggregate, and gunite surfaces.

Is it OK to Swim with Black Algae in the Pool?

If the slimy, mysterious black dots aren’t enough to keep you out of the pool, you may still want to reconsider cannon-balling in too soon.

Is Black Algae Harmful?

Black algae’s presence promotes bacterial growth and attracts insects. This could mean you’re exposing yourself to bacteria like E.Coli and running the risk of developing a rash and other illnesses. It is best to play it safe and avoid swimming until your pool is algae-free.

What’s the Difference Between Green Algae and Black Algae?

Identifying the type of algae in your pool can be tricky, so let’s take a look at the difference between the green and black strains.

Green algae are lighter in color and grow in large patches on the water’s surface or the pool’s walls. It grows relatively quickly but is easier to get rid of than other types of algae.

Black algae are darker in color and will appear in scattered spots across cracks and crevices throughout the pool. It is particularly difficult to get rid of due to its ability to seep into porous materials and cause structural damage that requires extensive repairs.

Recognizing Black Algae in Your Swimming Pool

While the presence of black algae may not be obvious at first, its rapid growth will soon make itself known. You will start to see black or blue-green spots or small clumps scattered across the pool that almost look like mold.

Don’t get too nervous at every black spot in your pool, though! There are a few more signs that will help you identify black algae.

Other Ways to Recognize Black Algae in Your Pool

  • The black and blue-green spots have raised heads.
  • It is attached to rough surfaces and will not freely float in the water.
  • It will not come off the wall easily with a pool brush or vacuum.
  • If you manage to scrape it off the wall, it will likely return time and time again without proper removal.

How to Treat and Kill Black Algae in Your Swimming Pool

So, how exactly do we remove every pool owner’s worst nightmare? Under the guidance of a professional team. Black algae are persistent, tough, and will often come back time and time again to wreak more havoc on your pool.

If you are experiencing a black algae problem, consult with a professional immediately.

Check Your Pool Water Levels

Begin by taking a look at your pool’s water levels. Test the water and adjust your alkalinity, pH, and chlorine levels to return to a proper balance. This will go a long way in helping when you shock your pool later on.

Clean Your Pool Filter

With a black algae growth, your pool filter has been dealing with a lot of nastiness. Remove and clean the filter cartridges with water or, if especially dirty, use a filter cleaner.

Consider replacing the cartridge completely if your filter looks a little worse for wear.

Brush Your Pool Thoroughly

Once grown, black algae have a strong hold on the surfaces of your pool. In order to fully loosen the algae from its rooted spot, you need to brush the pool thoroughly using a stainless steel brush. If you have a fiberglass or vinyl liner, use a nylon brush instead.

Take your time and put your back into it! This step is tough and requires a lot of effort, but the payoff will be worth it.

Scrub the Black Algae Spots and Brush Your Pool Some More

After you’ve finished with the brushing, it’s time to get to scrubbing. Look for areas with persistent black spots and scrub at them hard with a wire brush or putty knife.

Use Chlorine Tablets

Give yourself a little extra help and scrub with a chlorine tablet. Their rough surface will help break away particles, and the added chlorine will kill more algae.

Shock Your Pool

Now it’s time to shock your pool — preferably with a calcium hypochlorite shock. Shocking increases your pool's chlorine and other chemical levels and is a black algae killer.

Now, because black algae are so difficult to kill, we recommend using four times the recommended dosage for your size pool. Read the instructions before doing so, and if you’re unsure about the process, contact a professional for guidance.

Recommended shock dosage for black algae: 4LB of Calcium Hypochlorite for every 10,000 gallons of pool water.

When is the Best Time of the Day to Shock My Pool for Black Algae?

Because pool shock raises the chemical level of your pool’s water, it is unsafe to swim until the levels reduce back to normal. Always check your water levels after shocking before taking a dip.

It is also a good idea to shock your pool in the evening or at night. This way, the sun won’t affect the chlorine by dissolving it too quickly.

Turn On Pool Pump and Keep Running for 24 Hours

Turning on the pump will help evenly distribute the shock throughout the pool. Leave it running for at least 24 hours and let the shock do its job.

Add Algaecide

Next up, you’re going to add algaecide to the pool. Algaecide is a specialized product filled with additional chemicals that work toward killing the remaining algae in your pool. There are different types of algaecide on the market, so be sure to grab one specifically meant for black algae.

Brush Your Pool Again

Once the chemicals have done their work, you must put in a little more elbow grease. Begin by grabbing your stainless steel or nylon brush and brushing away just like before.

Shock Your Pool Again

Black algae are so resilient that we are going to recommend another quadruple dosage of shock. Repeat the process as before to make sure we kill every inch of algae.

Run Pool Pump Again for Another 24 Hours

Now that you’ve shocked the pool a second time leave the pump running for another 24 hours — we never said this was a quick fix!

Brush Some More

You read that right: it’s time to brush some more. After your second round of pool shock has settled, go over your pool once more, hitting any final missed algae spots with your brush.

Test Your Pool Water Again

Once all of this has settled, you can finally recheck your water levels. Adjust as needed to fall within the safe and recommended ranges.

Ideal Ranges

Here’s a refresher on what your water levels should look like:

  • Chlorine: 1.0-3.0 PPM (parts per million)
  • pH: 7.2-7.8 PPM
  • Total Alkalinity: 80-120 PPM
  • Calcium hardness: 200-400 PPM

Clean Your Pool Filter Again

After all that, your filter is in need of another round of cleaning. It will likely have black algae in it but don’t worry, the shock and algaecide have killed it!

Check for Black Algae and Shock and Brush Your Pool Again As Needed

While we hope these steps have rid your pool of black algae, it will always come back. Keep an eye on your pool and repeat the shocking and brushing process as needed.

What a Healthy Pool Looks Like Compared to a Pool With Black Algae

Remember how to recognize black algae! It will appear along rough surfaces and cracks of your pool in black or blue-green spots.

In comparison, a clean, algae-free pool will have clear water and clean floors and walls.

How to Prevent Black Algae in the Future

One of the best ways to keep black algae from growing in your pool is to keep up with regular cleaning and maintenance.

Other ways you can combat black algae include:

  • Cleaning your pool tools often.
  • Shocking the pool at least every week.
  • Check and replace your filter as needed.
  • Rinse off before swimming in the pool.
  • Maintain a regular vacuuming schedule.


Remember, even the cleanest of pools can fall victim to black algae under the right circumstances, and this alga works quickly to destroy your pool. If you notice black algae growing in your pool, it is important to speak with a professional as soon as possible for guidance. Find your local ASP - America's Swimming Pool Company and get in touch with one of our team members today.

Having trouble with other forms of algae like Green or Yellow? Check out our article: Pool Algae and How to Kill Any Type and/or Color

Disclosure: While basic pool care is encouraged for swimming pool owners, it is highly recommended you consult with a professional pool service like ASP - America's Swimming Pool Company for any issues like pool algae that arise.