How to Prevent and Get Rid of Black Algae in Your Pool the Easy Guide

Michael Keenan


Pool Troubleshooting

If you own a swimming pool, then you know that regular maintenance is essential for keeping everything clean and healthy. But one thing that many people don’t consider or expect is the presence of black algae in their pools. Black algae can be incredibly dangerous as well as difficult to get rid of.

In this article, I’ll explain exactly what black algae is and why it forms in swimming pools. Then I’ll teach you how to identify it in your pool and finally, how to remove it safely! Stick with me – it’s easy!

black algae in pool

Quick Answer:

Black algae is a type of bacteria found in pools and can be dangerous if ingested. It’s difficult to get rid of, but easy to prevent. Identifying it is key, as well as balancing the pool chemistry, brushing your pool surface regularly, cleaning your filters and using chlorine tablets.

If all else fails, use a shock treatment with granular chlorine and algaecide for best results. With regular maintenance you can keep black algae away from your pool!

What Is Black Algae?

Regular Algae is (most commonly) a green plant that grows in the water and belongs to a group that contains everything from micro organisms to seaweed. (See my guide on what causes algae). Its green color comes from chlorophyll, the pigment that helps it use light and synthesize its food from water and carbon dioxide.

Black algae is a type of bacteria called cyanobacteria which also contains chlorophyll which gives it a blue-green color. It also has other pigments which turn it black when mixed with the blue-green pigment.

This kind of algae can be dangerous as it blooms in water bodies while killing all other animals and organisms around it. It does so by blocking sunlight and using up all the oxygen and nutrients all organisms need for survival.

Black algae are known to harbor other harmful bacteria like E.coli and make toxins that are some of the most powerful natural poisons that can make anyone who enters the pool sick.

The severity of the illness increases if you ingest any of the infected pool water. Symptoms of such illnesses include nausea and stomach cramps and it can lead to liver damage.

Black algae are one of the most difficult bacteria to get rid of but one of the easiest to prevent. If you spot any of these organisms in your pool, you must act immediately.

But if you don’t act on time, they can be tough to get rid of since black algae grow into the cracks of the pool. They also look pretty menacing but you’ve got to do what you’ve got to do.

How to Identify Black Algae in Your Pool

Before getting ready to make your pool black algae free, it is good to identify what’s ailing your pool so that you can pick the appropriate course of action. Here’s how to do it.

Black algae typically grow in the areas of the pool that are in shade. They are a bigger problem for those who have pools with concrete or plaster finish. It is a great growing place for black algae because of the rough edges.

The top layer of the black algae is heavily slimy and accommodates skeletal growths on it which cannot be destroyed by regular chlorine treatment. This type of bacteria also doesn’t change the clarity of your pool water but creates black spots on top of it.

Now that you know what it looks like, here’s what you are supposed to do as treatment.

black algae on bottom of swimming pool

Prevent Black Algae in Your Pool

As mentioned before, they are tough to get rid of but easy to prevent. So, you might want to start there, if not now then for future reference. Here are a few tips.

  • Swimsuits that have been in the ocean have the tendency to carry back some of those little spores back into your pool. Be sure to bleach swimsuits after a dip in the ocean.
  • Make sure your pool is checked regularly so that the pH levels, alkaline and chlorine levels are balanced. They usually do a good job keeping the bacteria at bay.
  • Run your pump for eight to 12 hours every day so that any remaining spores are filtered off.
  • Make it a habit to brush the surface of your pool and vacuum it at least once a week to keep bacteria and debris to a minimum.
  • Keep an eye out for dirty pool equipment and toys. Sometimes, that is all it takes to get the pool dirty, no matter how much grooming you do.

As you can tell, this is all just good pool maintenance. As always this highlights the importance of good practice when managing your swimming pool. Get that schedule and stick to it – don’t be lazy!

Let’s say you messed up – it happens. You have a black algae problem in your pool, what can you do to get rid of it? That’s next.

How to Get Rid of Black Algae in Your Pool

Here’s how you can get rid of black algae in your pool. Just be aware you might need to repeat this process a few times to get rid of the problem totally. First, make sure you have all the supplies to get through the process without interruptions. Here’s what you will need.

  • Pool brush and pole
  • Pool shock treatment
  • Algaecide
  • Chlorine tablets
  • Granular chlorine

There are 10 steps in this process. Like mentioned before, it is not an easy task to get rid of black algae.

Step 1: Sanitize the Equipment

You must start the process by sanitizing all the tools that are to be used in the process so that you don’t accidentally introduce other harmful elements to your swimming pool. Clean the equipment by scrubbing it down with chlorine solution.

Step 2: Clean Your Filters

There is no doubt there are algae in your filters if they are already in your pool. You can try backwashing the filter. If you have sand or DE filters and your pool only has a small number of black algae, you can rinse the filter cartridge and get rid of it.

But if you don’t want to take a chance, you must use a filter cleaner as directed by the manufacturer. If that does not sound convincing enough, you can replace the filter medium or cartridge and get a fresh start.

Step 3: Balance the Water

Before you start cleaning the pool, it is recommended that you test the levels of the water. This helps you determine the amount of pool shock you need to use when you get there. Pool shock is also incredibly helpful in killing black algae.

You can check your pool water chemistry by using any of the test strips available in the market. Test the levels of pH, alkalinity and sanitizer. Calcium hardness levels are not a concern at this point.

Recommended levels are:

pH must be between 7.4 and 7.6, with 7.2 being the ideal value.
Alkalinity must be from 100 to 150 parts per million (ppm), with 125 ppm being the ideal value.

Chlorine must range from 1 to 3 ppm. If you use a different sanitizer make sure you know the optimum value for it.

Step 4: Scrub the Surface of the Pool

Once you have noted down the values of the water chemistry parameters, it is time to clean the surface of the pool. You can use a nylon brush for the sides and bottom of the pool.

Do this until all visible traces of the algae are gone. Then you must brush it again because they are likely to linger in your pool even if you can’t see them.

Depending on the severity of the problem, you might even have to use a stainless steel brush to make sure you got all the algae out of all the hiding places. If you have a fiberglass or vinyl layer, stick to nylon-bristled brushes.

Wait, we are not done yet. This step loosens the algae from the surface of the pool so that the shock can kill it effectively.

Need to remove sand or grit from the bottom of your pool? See my guide on how to fix that up now.


Step 5: Drop Some Chlorine Tablets

If you have scrubbed the pool to a point where you don’t really see black algae, very well done. But chances are, you will still see some spots. That’s them. Get those chlorine tablets you have kept aside. These will start killing the bacteria while you continue scrubbing.

At this point, you must put on protective gloves and eyewear. Break a three-inch chlorine tablet in half and hold the soft side firmly. Scrub on the black spots with the harsher edge of the tablet.

If there are spots in the pool that you can’t reach, get a telescoping pole. Attach the chlorine tablet holder to it. Place the tablet in the holder in a way that the broken part of the pill faces outwards. Scrub the spots while wearing your protective gear.

Let the chlorine enter the roots of the pool and clean it completely.

Step 6: Pool Shock Time

The best way to get rid of bacteria and algae is to give it a super shock with chlorine. Use a pool calculator or follow the manufacturer’s direction to the last detail for a pool shock.

Remember that a regular level of shock is not enough to destructure the DNA of the algae. And remember to do this at night because during the day, the sun will get to the chlorine before it cleans your pool.

Step 7: Introducing Granular Chlorine and Algaecide

After scrubbing away all the algae patches, generously add granular chlorine to those spots. Skip this step if you have a dark-colored pool.

Algaecide is a chemical made specifically to kill and prevent the growth of algae. Each bottle can treat 15,000 gallons of water.

Step 8: Run Your Pump

Give your pool 24 hours after the pool shock and run the pump for the next 24 hours. On any given day after that, you must run it for eight to 12 hours a day. This helps the shock disperse and do its job.

It is normal to find the pool water to be a bit cloudy after this.

Step 9: Brush the Pool, Again

The chlorine level will be pretty high after the pool shock. So, brush the pool surface at least twice again to make sure all the black algae is off the walls and the floor. You might want to do some light brushing for the next few days to make sure there is no lingering algae.

Step 10: Clean the Filter, Again

This is just to make sure there is no lingering algae in the filter that can come back into the pool. Wouldn’t that be an unfortunate turn of events?

As I said this is a pretty thorough process and will take some time and effort. You should also be prepared to repeat it all again in a few weeks to make sure you have solved the problem. Then it’s just a matter of sticking to a good maintenance schedule – like you should have from the start!

If you have repeated problems with the balck algae returning, I would suggest draining the pool and cleaning it thoroughly using the methods above. Then let it sit empty for a few days before filling and repeating the above steps once more.

black algae on pool tiles


What types of pool cleaning chemicals are effective in eliminating black algae?

Chlorine, bromine, and algaecides are effective in eliminating black algae from a pool.

How often should the pool be checked for signs of black algae?

The pool should be checked for signs of black algae at least once every two weeks.

How long does it take to get rid of black algae once it has been detected?

It usually takes several treatments over a period of time to get rid of black algae once it has been detected.

Are there any simple home remedies that can help remove black algae from a pool?

Yes, there are some simple home remedies such as using vinegar or baking soda solutions that can help remove black algae from a pool but they are not as effective as chemical treatments.

In Conclusion

It is a lot of hard work to get rid of black algae. That is why it is highly recommended that you come up with a pool maintenance schedule and stick to it so that your pool is free of gunk.

Taking care of your pool is important to keep it clean and free from black algae. Black algae can be difficult to remove but there are steps you can take such as regularly checking the pool chemistry, brushing the surfaces, and cleaning the filters.

It is also important to use chlorine tablets, granular chlorine and algaecide for a thorough clean. With regular maintenance and quick action when needed, you can keep your pool free from black algae.

michael keenan author

Author - Michael Keenan

I'm Michael Keenan the owner and creator of the Outdoor Care Guide. I'm a trained horticulturist with over 30 years of experience in pool care, plant care, and landscape care! Seemed like a good idea to share - I think I can make your life easier and save you some time and money!