How To Get Rid of Black Algae In Pool

By Shiloh McGinley

January 30, 2024

In this post, I'm going to go into detail on How To Get Rid of Black Algae In Pool, and prevent it going forward.

How To Get Rid of Black Algae In Pool


Black algae are nothing short of a nuisance for any pool owner to have. Growing with the help of poor filtration, water circulation, and chemical imbalance, they’re a nightmare to deal with and just as hard to keep from coming back. 

If by some unfortunate circumstance you find that you have black algae growing on the surface of your pool, worry not. It is completely removable though you’re going to need a lot of elbow grease.

In this article, we’ll discuss what black algae is, how it forms, and h0w to get rid of black algae in your pool!

What Is Black Algae?

What Is Black Algae?

Black algae is an aquatic organism that grows and multiplies in large colonies in the pool. Black algae are also called blue-green algae or Cyanobacteria.

They can grow on any pool type but are more likely to be found in plaster or concrete pools.

This is because of the naturally porous surface that makes it easy for the organism to take root.

What makes them more of a headache than normal algae is that they can grow even in pools that are well-maintained and regularly cleaned.

This is because they have a natural protective layer that is impervious to most pool-cleaning chemicals.

Although the black algae itself won't cause illness, it can attract insects or harbor organisms or harmful bacteria like E. coli, which can make swimmers sick.

Plus, no one wants to swim in a pool with slimy black gunk in it.

For information on other types of Algae, see our posts:

What Causes Black Algae?

Black algae usually come into your pool by being carried over by swimwear or objects from natural bodies of water like oceans, ponds, and lakes.

They do not naturally occur in a backyard swimming pool so it’s important to wash your swimwear and bathe before getting in the pool.

Once in the water, they naturally grow and multiply through poor filtration, imbalance of chemicals and lack of water circulation can help it spread faster. 

How To Get Rid of Black Algae In Your Swimming Pool

In all honesty, getting black algae off your pool and ensuring it doesn’t return won’t be a walk in the park.

But once it’s gone and preventive measures are in place, it’s likely never to return. To start dealing with black algae, you’re going to need the following tools:

  • Calcium Hypochlorite Shock (or Cal-hypo)
  • Pool Algae Brush (either steel or nylon)
  • Chlorine Tablets
  • Rubber gloves.
  • Pool Filter Cleaner
  • Putty Knife or Scraper
  • Pool Water Test Strips or a Testing Kit
  • Pool Filter Replacement (If required)

You should be able to find all of the above at your local pool supply shop, or my personal favorite shop

Next, follow the steps below:

1. Clean the pool filters.

Pool Pump & Filter

Since you’re going to be scrubbing off the algae from the wall, you want the filters to be nice and clean and filter all of this out effectively.

We recommend using a filter cleaner as opposed to clean waters since the latter is not effective against black algae.

Also, keep in mind that you’ll be cleaning these filters out after every time you scrub the walls.

The dislodged algae will likely be sucked into the filter so you want to clean them out regularly to avoid them creeping back into the pool’s walls.

If any black algae patches are growing on the filter itself, clean them with muriatic acid or a filter cleaner.

If you have sand filters or DE filters, then backwash these. If they are infected with black algae, throw them out and replace them with new ones.

2. Test the water chemistry

Swimming Pool Chemical Testing

With your testing strips or testing kits, test the water’s chemical levels and see if they’re balanced. If not, adjust the levels accordingly. 

This is because you are about to shock the pool and improper balance between pH levels and total alkalinity might inhibit that.

If the levels are not right, the hypochlorite Shock won’t work as effectively as it should.

For more reading on the above, see:

3. Begin scraping the black algae off

This is where the actual workout starts. With the putty knife or scraper, start scraping as much of the black algae off the pool’s walls as possible. Though this won’t get all of it out, it helps to get the bulk of the algae gone.

If the pool's surface is tile or vinyl, be careful with your pressure to note damage to the surface.

4. Brush off the black algae patches

Use your brush to then scrub at the black algae. You are trying to loosen them up and make them easier to remove later. 

After, with rubber gloves on, break a piece of chlorine tablet. With half the tablet, scrub at the black algae patches.

The roots of the black algae are long and embedded into the surface of the pool.

What you’re trying to do by scrubbing chlorine directly onto it is kill the algae at its roots. You can also use a chlorine tablet holder to do this

5. Triple-Shock The Pool Using Calcium Hypochlorite

How to Shock Your Pool

You need to add a triple dose of the regular amount of calcium hypochlorite pool shock to the pool water.

You’ll need 3 pounds of calcium hypochlorite per 10,000 gallons of water. Some want to be extra sure and add 4-times the amount.

Shock the pool at night so the chlorine won’t be depleted by the sun’s UV rays. You can also put the pool brush and other cleaning equipment in the pool while shocking to clean them.

For a step-by-step of this process, see: How to Shock A Pool

6. Run the pumps.

Once you’ve added the chlorine, run the pumps for a few hours to let the chlorinated water filter and circulate in every area of the pool.

Once enough time has passed, turn the pumps off and let the pool set overnight.

7. Repeat steps 1-4 afterward

Repeat the process of scrubbing at the walls of the pool to dislodge any black algae left.

Once dislodged, they will either be picked up by your pool brush or fall into the pool’s water where the high levels of chlorine will kill them.

After, clean the filters again of any black algae that it might have caught. Use scrubbing, filter cleaning products, and/or muriatic acid to ensure as spotless a clean as possible.

8. Shock the pool for the last time

For safety's sake, shock the pool again but this time, use only a single dose of calcium hypochlorite in the water.

That’s 1 pounds per 10,000 gallons of water. If you see any more black patches still clinging to the walls, double this dosage.

Like before, run the pump again for a few hours to circulate the chlorine properly.

9. Repeat step 1-4 as needed

This time, scrub the pool walls as hard as you can. Usually, you shouldn’t see any black dots at this point but black algae are very stubborn and you want to be extra certain. If you notice any black dot, scrub it with a chlorine tablet again as you did earlier.

10. Retest and readjust water chemistry

Measure The Chemicals In Your Swimming Pool

Due to all the shocks and scrubbing you’ve done to the pool, it’s likely that the chemical levels are off.

Once you’re done with the process and certain that all the black algae have been dealt with, retest the pool’s water with the strips or testing kit. Adjust accordingly should you find any imbalances.


The main and surest way to prevent black algae from coming back is to thoroughly wash any clothes, toys, floaters, or pool equipment that had contact with a natural body of water.

Remember that black algae does not form in the pool by itself but is introduced to it by someone who has recently been to a lake, river, or ocean.

You can also ask other swimmers to wash before getting into the pool too.

Apart from that, be sure to keep up any pool maintenance practices as much as you can.

This means testing the water’s chemicals at least once a week, allowing the filter to run for at least an hour, and scrubbing the pool’s walls and floor regularly.

Also, ensure all the pool equipment you use is sanitized beforehand and that the filters are backwashed or replaced weekly.

Finally, shock your pool immediately after seeing any algae on the surface. 


Black algae is a beast of an organism to deal with. The scariest thing about it is that you can never be too certain that you got everything out.

Little that you know, there might be little microscopic roots still lingering on the surface just waiting to grow into a new patch (and make life harder for you).

Though the steps are hard and labor-intensive, any other way won’t kill it completely. In the worst case, it grows out of hand and leaves your pool entirely unswimmable.

That’s why we recommend that you scrub, shock, and filter out as many times as you need to be certain.

Like we said in the beginning, though black algae are hard to deal with, once it is gone it is gone for good. 

Shiloh McGinley

About the author

Shiloh McGinley has been in and around swimming pools her whole life. She's seen a lot of products come and go, and she wants to share with you the best products that really work!

Shiloh is passionate about helping people stay safe in the water, and she loves educating others on how to choose the right pool products.

When she's not working, Shiloh enjoys spending time with her family and friends, and swimming - of course!

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