How To Remove Black Algae From Your Pool

Pool Care Chapters

There are three methods commonly practiced to remove black algae. They are all effective, but none of them will remove 100% of the black algae from your pool. Black algae are extremely difficult to exterminate. Once you have it and have dealt with it, you will still need to keep a close watch on your pool to step in if there is a resurgence.

We have written in-depth, step-by-step instructions for the first method, scrubbing and shocking. Scrubbing and shocking is the most recognized and recommended method to remove black algae. We have given some notes for the other two methods, but those topics deserve complete separate guides of their own. For now, we’ll give you some basic information to get you started and update the article when we have more to say.

Method 1: Scrub and Shock

Method 2: Replaster and Retile

Method 3:  Acid Wash, Bleach Wash, and Pressure Wash

Method 1: Scrub and Shock

Equipment Needed

  • Algae Brush
  • Pool Shock (Calcium Hypochlorite is best for algae)
  • Chemically resistant gloves and goggles
  • Chlorine tablets
  • Filter cleaner
  • Telescopic pole
  • Test strips or Liquid Test Kit
  • Depending on which type of filter you own, you will need a backwash hose, DE powder, filter sand, or a replacement filter cartridge.

You will likely need to scrub down and quad-shock your pool twice to remove all of the black algae from your pool. During this process, take some time to ponder the potential sources of black algae. Black algae do not arrive from improper maintenance but are carried from natural bodies of water to your pool.

That algae often clings to clothing, gear, towels, and toys, so you should clean and replace belongings as you deem necessary. In the future, we highly recommend that you wash these belongings after visits to natural bodies of water.

Black algae can also attach to people. While you obviously can’t replace them, you can require soapy showers before entering the pool. If you don’t take these precautions, there is a good chance that you’ll be doing this all over again very soon.

Step 1: Clean the Filter

If black algae has just begun to show its bacterial face, but it hasn’t overtaken your pool, rinsing or backwashing might be enough to clean the filter. If a significant portion of the pool is speckled with black mold, you’ll need to take more aggressive action. Depending upon the filtration system you have, use a filter cleaner, replace the filter’s medium, or replace the cartridge.

Step 2: Test and Balance the Water

Before treating the black algae, you should get your pool’s chemical balance in order. Chlorine is most effective when alkalinity, pH, and sanitizer are within specific ranges. Continue to step 3 when your pool’s chemicals test between the following parts per million (PPM) values.

  • Alkalinity: 100 to 150 PPM
  • pH: 7.2 to 7.4 PPM
  • Chlorine: 1 to 3 PPM

Step 3: Scrub the Whole Pool

Black algae’s skeletal growths are covered in a slimy protective coating. That coating protects it from the chlorine’s typically effective ability to oxidate and mutate the proteins of biological contaminants. We need to remove that coating so the chlorine can complete its deadly work.

Removing that coating will take a lot of scrubbing with a brush with firm bristles. As you scrub, the black algae will flake off, and those flakes will make their way to the pool’s filtration system. The exact type of brush you’ll want will depend on the type of pool you have:

  • Concrete, Gunite, or Plaster: Stainless steel bristles
  • Fiberglass or Vinyl-liner: Nylon bristles

Until step 4, you don’t need to focus on the heavily infested black spots. We’re more concerned with scrubbing the surface of the entire pool. 

The smallest particles of algae are too tiny for the human eye to perceive, and they’ll attach to the floor, walls, and crevices. We want to get those off of the pool’s surfaces before we start worrying about the trouble spots.

Step 4: Scrub the Black Spots

Now that you’ve given the whole pool a good scrub, it’s time to give some special attention to all of the black algae spots that you can find. First, give those algae spots an extra hard scrubbing with a brush.

When that’s done, if you have a concrete, gunite, or fiberglass pool, it’s time to put on goggles and some chemically resistant gloves. You’ll need to break a chlorine tablet in half and scrub the flat end of one of those halves upon the black algae. 

Step 5: Scrub Some More

When every black spot has been scrubbed with the chlorine tablet, it’s time to do even more scrubbing. We know, it’s a lot. Black algae are pervasive and persistent. It takes a lot of effort to remove. We need that elbow grease to make this method work.

Step 6: Quad-shock Your Pool

This is the first pool shock of possibly two pool shocks you will need to perform. We won’t give the step-by-step instructions of a pool shock because we’ve already written a guide. If you want those instructions, we recommend heading to our guide, “How to Shock a Pool.” 

Day-to-day chlorine levels should be between 1 to 3 PPM. A standard pool shock will raise those levels to 5 to 10 PPM. We need a quad-shock so that the pool shock will be four times as powerful as a standard shock.

With that said, you must be cautious. Chlorine levels become hazardous above 6 PPM, dangerous above 14 PPM, and lethal above 34 PPM. Chlorine kills algae at PPM levels between 10 to 20 PPM. 

Don’t take unnecessary risks. Let your household know that the pool isn’t safe, and nobody should take a dive in the pool while the chlorine is high. As a precaution, It would be a good idea to mark the pool as being unsafe.

Tip #1: Calcium Hypochlorite

Calcium hypochlorite (Cal-hypo) is an effective chlorine choice because it releases lots of calcium and free chlorine into the pool. When exposed to calcium, black algae’s boney shell becomes softer and more fragile. In other words, it becomes easier to demolish. After that, all of the free chlorine can bind to the algae and destroy it.

That said, some downsides might make another choice better for your pool. Calcium hypochlorite can cause colors in certain pool surfaces to fade. Those surfaces include fiberglass, painted, pebble tec, and vinyl.

Additionally, while the calcium helps soften the black algae’s shell, it will throw off your pool’s calcium content. When the black algae have been killed and filtered, you will need to rebalance the pool chemicals.

Tip #2: Start the Pool Shock at Sunset

The chlorine levels will be much higher than the chlorine stabilizer will be able to handle. That’s a good thing since the levels are hazardous. However, it does mean the chlorine will rapidly evaporate as soon as the sun comes out. 

Some choose to perform their pool shock for a total of 24 hours. As the levels drop, they keep adding more chlorine. However, we think this is a mistake. In direct sunlight, the evaporative rate of chlorine is exceptionally high. You would need to replenish the chlorine every hour, which is both wasteful and annoying.

Instead, we believe you should perform a quad-shock at sunset and do it again the next evening if necessary.

Tip #3: Toss Your Pool Equipment into the Pool

Right now, the quad-shock presents an excellent opportunity to clean your pool toys and equipment easily. Toss any submersible pool gear into the water to cleanse them of any black algae that might have attached itself to the equipment.

Step 7: Run the Pool Pump for 24 Hours

Turn on the pool pump and let it run for 24 hours. The pump will disperse the chlorine throughout the pool and filter out the black algae. Don’t be surprised if you come back to a cloudy pool. That’s all expected, and it will eventually clear up on its own.

Step 8: Scrub the Pool Again

While the pool is saturated with chlorine, you should do some more scrubbing throughout the entire pool. However, if the scrubbing will get you wet, we recommend wearing goggles, acid-resistant gloves, pants, and a waterproof jacket. 

According to the CDC, it’s dangerous to be exposed to chlorine levels between 14 to 21 PPM for 30 minutes or more. To kill algae, we need your pool’s chlorine to be between 10 and 20 PPM. While you might not be going for a 30-minute swim, it’s best to play it safe. 

Chlorine kills contaminants by mutating protein structures. Cancer often begins with mutations to the protein structures that make up our genetic codes. While there is no proven link between chlorine and cancer, it’s best not to take any unnecessary chances with your health.

Step 9: Clean the Filter Again

All that scrubbing has sent a lot of black algae into those filters. Algae is persistent, so you’ve got to clean them out. Depending upon your filter type, you’ll need to backwash them, rinse them, or clean them.

Step 10: Check the Pool for Black Algae

We already know that black algae spores are tiny. However, they are so tiny that they can pass through porous surfaces. Those spores are inaccessible and cannot be scrubbed. 

While you might not be able to reach them, chlorine will eradicate a portion of those spores. However, some will almost assuredly be left. The black algae will likely make a return from what remains within those inaccessible regions. 

Step 11: If Needed, Repeat Quad-Shock and Scrub

If there are still black algae spots present, you already know what to do. Bring the chlorine levels up to 10 to 20 PPM and scrub away. If you don’t see any black algae, you can move onto step 11. That said, even if you don’t see any spores, it might be a good idea to perform a quad-shock to ensure the best possible chance to eliminate this threat to your pool. Human eyes are too limited to perceive the smallest black algae particles, so relying on them would probably be a mistake.

Step 12: Carry Out a Pool Chemical Test

If you are satisfied with the results, perform a pool chemical test. If the chemicals are within their ideal PPM ranges, move onto the next step. If they are outside of those ranges, bring the pool chemicals back into balance.

Step 13: Monitor the Pool Over the Next Couple of Weeks

There is a decent chance that the black algae will try to make a comeback. If it does, you know what to do. Scrub with a brush, scrub with a chlorine tablet, and shock if necessary

Method 2: Replaster and Retile

Even though this is the most effective method, we don’t recommend it unless it is already time to replaster your pool. Typically, replastering only needs to be done once every ten years. According to Thumbtack, replastering costs an average of $4 to $7 per square foot and $5,440 for the entire pool. That is an expensive solution to a problem that you can mostly straighten out with a bit of elbow grease and a few days.

Replastering will seal the black algae. The algae are still there, but it’s hidden away and made mostly irrelevant. 

It might be the most effective solution, even though it doesn’t technically eliminate the algae.

Since algae can infiltrate permeable surfaces, pool tiling can harbor algae within the porous grout. For that reason, if you choose to replaster, you should also retile. If you don’t, you’ll risk the black algae’s return.

You should also clean items with chlorine and replace porous pool items potentially infected with the black algae spores.

If this method appeals to you, we recommend that you consider re-plastering your pool.

Method 3:  Acid Wash, Bleach Wash, and Pressure Wash

This is a simple but effective method. We only recommend it if black algae keep coming back. The downside to this method is sacrificing some of the plaster to this very corrosive acid. This will reduce the life expectancy of that plaster. On the upside, it is much cheaper than method 1. Also, it isn’t as physically taxing as method 3. 

You will need to gather several pieces of protective equipment, a pressure washer, and chemical components for the acid and bleach wash.

It does come with some serious risks to your health and the health of your pool. You’ll need to wear the appropriate protective gear, and the pool should be brushed and washed almost immediately after exposure.

The risks and gear shouldn’t be a big surprise when you consider that we are talking about the acid that will kill black algae because chlorine couldn’t.

Like the replastering method, the tasks in this method are more useful than their applications for black algae. For that reason, we will simply say to acid wash, bleach wash, and pressure wash your pool. When we have written these guides, we will update this paragraph with the appropriate links. In the meantime, we will simply provide links to Google searches to help you find the guides that you need.

Links to Google Searches

How to Prevent Black Algae in the Future

If the pool was exposed to black algae, it will likely grow even if your pool chemicals are balanced and the filtration system is working as it should. In other words, Black algae did not infest your pool because you did something wrong in your daily maintenance. It likely appeared because black algae were transported from a body of water by way of swimwear, toys, or gear.

To prevent this in the future, sanitize everything after you’ve visited a natural body of water or a pool with past or present algae issues. If the algae happened once, it would likely happen again without taking those precautions.

For the toys and gear, you can use your average spray cleaner.

If you don’t have that on hand, a tablespoon of bleach in a gallon of water will work just as well.

Run your pool pump every day, and consider upgrading to a better filtration system. The majority of pool filters aren’t capable of filtering the small 2-micron width of algae particles. Some claim that they can filter those small algae particles, but some companies exaggerate the capabilities of their filters. It would be a good idea to research user experiences before making any purchases. 

We have done a bit of research into filters and their capabilities and written up a report for those looking to upgrade. In that article, we didn’t review any particular product but gave a description of the filter types, what they are capable of filtering, how to increase those capabilities, and other benefits and disadvantages of each type of filter. If that interests you, you should read “Best Pool Filter Type: D.E., Sand, or Cartridge.”

You should also scrub and bleach any parts of the pool that might be contaminated. That includes the diving board, ladders, or solar blankets that might have contacted the algae.

Algae is one reason your home’s pool rules should include the swimmers and their bathing suits taking a shower before hopping into the pool.

What is Black Algae?

There is a common misconception regarding Black Algae. It isn’t an alga at all. In reality, it is a type of bacteria called cyanobacteria or blue-green algae. The incongruent names aside, it is still the same enemy. 

The coloration is the result of the light-capturing components that algae need to perform photosynthesis. The first is the well-known chlorophyll that gives plants their green pigmentation. The second and darker coloration is a result of water-soluble pigments called phycobilins. 

Black algae is an intrusive and prolific species of bacteria that can survive harsh environments, so it can be challenging to remove. It goes dormant in cold winter temperatures. It can survive, for a time, without any source of water. Finally, you can reduce it to a few particles, and it will regenerate back to full strength over time.

How harmful are black algae? Can black algae kill you?

It’s not impossible, but it is also unlikely. Currently, there isn’t any reliable data connecting black algae from a pool as a direct cause of death. The only confirmed case of a semi-direct cause involved an individual with a highly compromised immune system who was on kidney dialysis. Healthy individuals do not seem to have to worry about lethal implications based solely on confirmed reports. That doesn’t mean that there isn’t a risk.

When we look deeper, the water becomes a little murky. Technically, It isn’t the black algae that is toxic. The cyanotoxins that they produce are the dangerous component of this equation. There are several species of cyanobacteria (blue-green/black algae), and they produce different cyanotoxins. According to the CDC, some of those toxins can be deadly, and they are listed as the most powerful natural poisons in the world. We also don’t have a remedy for these poisons. While a deadly poison sounds bad, jumping into a pool of black algae doesn’t appear to be enough to cause lethal issues for humans.

What Are the Symptoms of Black Algae Poisoning?

While death isn’t a likely scenario, illnesses are. They can be contracted through inhalation, ingestion, or swimming through black algae waters. According to the CDC, common symptoms of these illnesses include:

  • Blisters
  • Fatigue
  • Nausea
  • Stomach cramps
  • Liver damage
  • Rash
  • Weakness
  • Vomiting

Those are the most common symptoms, but there are many more. The expected symptoms change with the method of exposure. If you want to know more, we highly recommend that you head to the CDC page that goes into much greater detail about the health-related aspects of cyanobacteria. On this page, we have primarily focused on how to get rid of it.

Don’t Let Your Pets Drink the Water

While humans do not make a habit of drinking pool water, pets do not always know better. They will drink and swim through waters containing cyanobacteria blooms. There are many reports of dogs drinking fatal amounts of cyanotoxins, so don’t let this problem bloom and grow.

If you want to be 100% certain of what you are facing, you can always purchase a blue-green algae testing kit from Amazon. This particular kit gives results in 15 minutes, and it’s intended to inform dog owners if the natural waters are dangerous to their dogs. It should work just as well to identify blue-green algae (A.K.A. Black algae) in your home pool.

Questions and Answers

How Long Does It Take to Kill Black Algae?

It depends on the method used to kill the algae. If you shock and scrub, it will take about 3 days. If you retile and replaster, it will take one day. If you acid wash, chlorine wash, and pressure wash, it will take one day.

Does Shock Kill Black Algae?

Yes, pool shock does kill black algae. However, you’ll need the chlorine to reach levels between 10 to 20 PPM to be lethal to algae. Be extremely careful. We do not recommend raising the chlorine to 20 PPM. High chlorine levels are risky to animals and people that might ingest or swim in the water.

What Is the Best Algaecide for Black Algae?

Copper algaecides are your best bet, but be aware that some of them can stain concrete pools. If you have a concrete pool, keep an eye for copper algaecides specifically labeled as non-staining. Certain algaecides can kill black algae, but they are best used as a preventative measure. 

Algaecides disrupt algae’s ability to photosynthesize by limiting its ability to capture sunlight. They also disrupt the electron transfer required to set off the whole process. Without a steady source of food, it is difficult to multiply.

The Pool Care Handbook & Video Course

by Swim University
This is an illustrated e-book with detailed videos and step-by-step instructions on how to best care for your swimming pool. 
© 2023 Pool Care Guide. All rights reserved.