Of all the organisms that can negatively affect your pool, black algae is the worst of the lot.
So what is black algae? Why does black algae appear in a pool? And how can you get rid of it? The answers to all these questions and more will be revealed in this post.
Let’s get to it.
Article Contents
What is Black Algae?
Black algae is an aquatic organism that grows and multiplies in large colonies in the pool. Black algae is also called blue-green algae or Cyanobacteria.
This organism, like other aquatic plants, contains chlorophyll which gives it a green tint. But black algae also contains water-soluble pigments that mask the green color, hence the blue-green or black appearance.
Black algae thrives in both sun and shade and it appears as black spots or dots on the walls of the pool.
Black algae or Cyanobacteria is more common in areas of the pool made from concrete and plaster. This is because these materials are porous and the black algae can grow roots in them.
Black algae is also the toughest type of algae to get rid of. This is because the organism develops strong and long roots that can penetrate pool walls making it impossible to brush off completely and difficult for chlorine or algaecides to penetrate it.
The organism can even grow and multiply in pools that are well-maintained and sanitized. This is because black algae has a natural protective layer that prevents pool chemicals from having effect on it.
So how do you know if you have black algae in your pool? Let’s find out.
How to Identify Black Algae in Pool
Black algae is much more likely to occur in concrete or plaster swimming pools. If you have a fibreglass, vinyl liner or inflatable pool, like an Intex pool, you’re much less likely to have a black algae problem.
The followings signs indicate the presence of black algae in a pool:
- Blue-green or black spots on the pool’s surface
- Raised non-floating mold-like heads in the pool water
- Black patches on rough and porous areas in the pool
- If the black spots don’t brush off with pool brush
- If the black spots can be scrapped off pool walls
Now, let’s check out how these signs indicate black algae presence in the pool.
Blue-green or Black Spots On The Pool’s Surface
If you notice multiple black spots in or on the pool’s surface, that’s an indication of black algae in the pool.
Ordinarily, a single black spot on the pool’s surface isn’t a sign of black algae. Most times, a single spot could be a mineral stain. But if the spots or dots appear in large numbers, then it’s black algae. This is because black algae grows in colonies.
Raised Non-Floating Mold-like Heads In The Pool Water
If you notice raised mold-like black heads in the pool water, it’s an indication of black algae. Black algae don’t float around in the water because the roots keeps them attached to the pool’s walls and floors. Instead they have a raised head in the water.
Black Patches on Rough and Porous Areas in The Pool
Black algae thrives in rough and porous areas in the pool. So any black spot in such areas should be treated as black algae.
The porous areas made from plaster and concrete especially are home to black algae because the roots can easily penetrate into these areas.
If The Black Spots Don’t Brush Off
Black algae is very difficult to remove even with a pool brush. So if you notice dark patches that don’t come off after scrubbing, there is a high chance it’s black algae.
If The Black Spots Can Be Scraped Off
Black spots caused by mineral stains can’t be scrapped off the pool walls. So if you notice black patches in multiple numbers that can be scrapped off, it’s usually black algae.
Now you know how to identify black algae in the pool. But what causes black algae to appear? Let’s take a closer look.
Related Reading: What to Do if Swimming Pool Filter Not Filtering Algae
What Causes Black Algae In Pool?
Black algae or blue-green algae in pools is usually transferred from natural bodies of water like oceans, ponds, and lakes by clinging onto swimwear. If the swimwear is hasn’t been washed before using a swimming pool, the algae can be transferred into the pool.
Black algae doesn’t naturally occur in backyard swimming pools. Unwashed swimwear is the primary way it is transferred.
Once inside the pool, it can multiply quickly.
What Causes Black Algae to Multiply and Grow?
The following factors aren’t causes of black algae but they can increase the rate at which black algae multiplies in the pool.
Inadequate Water Circulation
This won’t cause black algae but it can cause the organism to multiple quickly.
Black algae thrives in warm water that is under-circulated or stagnant. So if you are not running the pool pump regularly, black algae will multiply quickly since the pool water isn’t being circulated.
Damaged Pool Filters
The filters in your pool are responsible for trapping foreign contaminants like black algae that enters the pool. But if the pool filters are damaged, black algae will grow and multiply freely in the pool.
Unbalanced Pool pH and Not Enough Sanitizer
An unbalanced pool pH and lack of sanitizer will make it possible for black algae to multiply in the pool.
When the pool pH and water chemistry is unbalanced, the effectiveness of pool chemicals like chlorine will be reduced. This will allow black algae bloom. pH should be between 7.4 and 7.6.
And for pools to kill off all bacteria, it’s essential the correct level of chlorine is maintained – 1-5 ppm with 3-5 ppm being ideal. If you have a saltwater pool, it’s possible the chlorinator isn’t working correctly. Check out: 13 Reasons Why Saltwater Chlorinator IS Not Working or Generating Chlorine
So now you know the cases of growth and spread of black algae in pools. Now how do you get rid of black algae?
How To Get Rid Of Black Algae
Getting rid of black algae is difficult but not impossible.
What You Need
To remove black algae from a pool, you’ll need the following supplies:
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 | Clorox Pool&Spa XtraBlue 3" Long Lasting Chlorinating Tablets 25 lb
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Here is a quick rundown of how to get rid of black algae from a pool:
- Rinse The Pool Filters in Pool Filter Cleaner
- Test and Adjust The Water Chemistry
- Scrape The Black Algae off The Pool Walls
- Brush The Black Algae Patches
- Brush The Black patches with Chlorine Tablets
- Triple-Shock The Pool Using Calcium Hypochlorite
- Run The Pool’s Pump
- Brush The Black Algae Patches Again
- Rinse The Pool Filter Again
- Shock The pool
- Run The Pool’s Pump
- Brush The Pool Walls
- Retest and Readjust The Pool Water Chemistry
Now, let’s get to work.
1. Rinse The Pool Filters in Pool Filter Cleaner
The first step is to clean the pool filters. Since there is black algae in the pool, chances are there will be more black algae trapped in the pool filters. So you need to clean the pool filters first.
Though pool filters can be rinsed in clean water, it’s better to clean the filters in pool filter cleaner. This is because clean water alone is not enough to get rid of black algae in the pool filters.
Cartridge Filters
If there are many black algae patches in the filter, you should soak the filter in filter cleaner for a few hours and then scrub the filters after. You can also use muriatic acid to clean your filter:
Further Reading: How to Clean Pool Cartridge Filter (Using Muriatic Acid)
Sand Filters & DE Filters
For sand and DE filters, you can backwash these. Make sure you backwash to waste. If you have a lot of black algae, it may be worth replacing the sand or DE material.
2. Test and Adjust The Water Chemistry
The next step is to adjust the water chemistry. Black algae is usually removed by adding a high dose of calcium Hypochlorite Shock to the pool water.
But before you can Shock the pool, the water chemistry has to be at the right level. If not, the calcium Hypochlorite will not work.
You can test all these levels using a pool test strip or pool water testing kit. Ensure the pool chemistry is within the ideal range before going on with the task.
pH Levels
The pH level of the pool water should be between 7.4 and 7.6 on the pH scale. You can adjust the pH level using pool pH reducer or pH increaser.
Stabilizer Levels
Stabilizer levels should be between 30-50 ppm. Add cyanuric acid to increase this. To decrease, you’ll need to partially drain the pool and refill it to dilute it.
Chlorine Levels
The active chlorine level should be between 1 ppm and 5 ppm with 3-5 ppm being ideal. You can add more chlorine to increase the chlorine level of the pool.
Alkalinity Levels
The alkalinity should be between 100 ppm and 150 ppm. You can adjust the pool alkalinity using alkaline increaser or alkaline reducer.
Related Reading: How to use Pool Chlorine Test Kits
3. Scrape The Black Algae Off The Pool Walls
Black algae though tough to remove completely can still be scrapped off the pool walls. Scraping the black algae off the walls first helps to remove bulk of the black algae heads so the calcium-hypo can be more effective.
Use a putty knife or scraper to scrape off the black algae heads from the pool walls.
If you have a fiberglass or vinyl liner pool, you should be careful with the scrapping intensity. Also, don’t use sharp-edged metallic objects on pool walls on vinyl liner pools.
Don’t worry if you can’t scrape all the algae off. The calcium hypochlorite will take care of the rest.
4. Brush The Black Algae Patches
Use a steel-bristled pool brush for concrete pools, or a nylon brush for fiberglass or liner pools, to scrub the spots where the black algae is. Brushing these spots will loosen the grip that the black algae has on the pool walls, so it’s easier to remove. You’ll need to use quite a lot of effort when scrubbing.
As stated earlier, black algae has strong and long roots that run deep into the pool frame. So even after you have scraped and scrubbed the black algae, chances are the roots will still be inside the pool walls.
If you paid attention in science class, then you know that if the roots are still alive, the black algae can still grow.
To kill the roots, you should …
5. Scrub The Black Patches With Chlorine Tablets
Put your gloves on and break a chlorine tablet into two. Then use one half to scrub the black patches on the pool surface.
What you are doing is directly rubbing chlorine on the black algae spots. This will stop the roots inside the pool frame from growing.
You can also use a chlorine tablet holder to do this. It’s a holder that fits on the end of a pool pole and is designed to hold regular chlorine tablets. That way you can scrub without getting wet!
Here’s a chlorine tablet holder:
- Allows chlorine to be rubbed on algae & stains
- Compatible with most telescopic poles
- Holds any 3-inch swimming pool tablet
6. Triple-Shock The Pool Using Calcium Hypochlorite
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.
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.
7. Run The Pool’s Pump
After adding the required amount of calcium hypochlorite to the pool water, turn on the pool pump and let it run for a few hours.
Running the pool pump ensures the calcium hypochlorite is circulated in every corner of the pool. After a few hours, turn off the pool pump and leave overnight.
Related Reading: How Long Should I run my Pool Pump After Shocking?
8. Brush The Black Algae Patches Again
What’s next? Another round of scrubbing. Use the pool brush to scrub the pool again. Pay attention to the spots where there are black algae dots or patches.
As you scrub the pool walls, the black algae will fall into the water where the calcium hypochlorite will kill it. Scrubbing will also help to expose the root making it more vulnerable to the chlorine shock.
9. Rinse The Pool Filter Again
Don’t get fed up, you are almost done. The first rinse of the pool filters was to get rid of any live black algae in the filtration system.
But since you have scrubbed the black algae patches, shocked the pool, and switched on the pump, there will be more dead black algae heads in the filtration system.
So clean the filters again in filter cleaner. A bit of scrubbing will help.
10. Shock The Pool
Yes, you need to shock the pool again. This time, add a single dose or calcium hypochlorite to the water. That’s 1lbs per 10,000 gal of water.
If you notice many black patches still clinging to the pool surface, you should double the regular dose of calcium-hypo.
11. Run The Pool Pump
By now, you should know that whenever you shock the pool, you have to run the pool’s pump to ensure circulation. So do just that. Let the pump run for a few hours.
12. Brush The Pool Walls
What’s next? You got it. Another round of scrubbing. Keep pushing, you are almost there.
This time, scrub the pool walls as hard as you can. Usually, you shouldn’t see any black dot at this point but black algae is very stubborn. If you notice any black dot, scrub it with chlorine tablet again as you did earlier.
13. Retest and Readjust The Water Chemistry
The repeated scrubbing and adding of chemicals to the pool might have offset the water chemistry. So retest and readjust the alkalinity and pool pH.
After getting rid of the black algae spots, keep tabs on the pool for a few days to see if any black head will pop up again. If it does, shock and scrub the pool yet again.
You can also use algaecides to get rid of mild cases of black algae.
Best Algaecides For Black Algae
Here are some of the best products to choose from:
Bio-Dex Algaecide
This product from Bio-Dex is designed to kill and prevent stubborn algae, like black algae. It also works for other types of algae and the best part is it doesn’t affect any part of your pool chemistry.
Here’s the McGrayel algaecide:
- Kills and prevents stubborn algae
- Works for other types of algae
- Doesn’t affect pool chemistry
Pool RX Black Algaecide
This product from Pool Rx is a top choice when it comes to black algaecides due to its lasting potency.
The algaecide reduces pool demand for chlorine and it retains its effectiveness for up to 6 months. It also works on other types of algae.
Here’s the Pool RX Black Algaecide:
- Eliminates all types of algae
- Reduces Chemicals
- Effective for up to 6 months
HTH Super Algaecide
This product by HTH is designed to kill and prevent the growth of black algae. What’s more? The product can be used directly from its container which makes it user-friendly.
It is also safe to use on vinyl liners and fiberglass pools. It doesn’t corrode pool equipment and the pool can be used in less than an hour after using the product.
Here’s the HTH Super Algaecide:
- Kills and prevents all algae types
- Great for vinyl-lined and salt water pool systems
- Non-foaming algaecide
How To Prevent Black Algae
The best way to prevent black algae in a pool is to wash swimwear, toys, floaters, and other equipment that you may have used in natural bodies of water before using them in the pool.
This will prevent black algae from growing in the pool. But to keep black algae from multiplying in the pool, you should always:
- Test and adjust the water chemistry
- Shock and scrub the pool walls and floor if you notice any signs of algae.
- Rinse or backwash the pool filters weekly
- Scrub and wash the pool walls frequently
- Run the pool’s pump for at least 8 hours daily
- Keep all pool equipment clean and sanitized
- Add the required dose of pool chemicals when due
- Ask every swimmer to take a shower before getting in the pool.
Now you know how to get rid of and prevent black algae growth in the pool. But what are the dangers associated with swimming in pool with black algae? Let’s take a closer look.
The Dangers or Black Algae (Blue-Green Algae)
Is Black Algae (Blue-Green Algae) Dangerous for Swimmers?
Black algae or blue-green algae in swimming pools is a health risk for swimmers. Coming in contact with it or swallowing it can cause nausea, cramps, muscle weakness and irritations of the skin and eyes.
Cyanobacteria (blue-green / black algae) is also widely known for producing cyanotoxins in the pool which is a potent poison that is harmful to swimmers and pets.
Is it safe to swim with blue-green or black algae?
You should hold off swimming if you have black algae in your swimming pool. It may not be safe and may cause health issues. Treat the algae by super chlorinating (shocking) it and scrubbing the algae before swimming in the pool.
Is blue-green or black algae dangerous for pets?
Blue-green or black algae can be deadly for pets. Pets like dogs should not be allowed to swim in water when blue-green or black algae is present.
Is Black Algae or Blue-Green Algae Bad for Pools?
Black algae roots can cause cracks and dents in pool walls which can in turn cause leaks. Black algae patches also stain pool walls, making the pool uninviting to swimmers.
Other Dangers of Black Algae
The presence of black algae in the pool can attract insects like mayflies to the pool too. Accumulation of dead mayflies for instance is known to cause hay fever and other allergies.
Black algae also cause harmful bacteria like E.coli to grow in the pool. If pool water with E.coli is accidentally swallowed by swimmers, it can cause nausea, stomach cramps, and vomiting.
You can learn more about the dangers of black algae here.
Final Thoughts
Overall, having black algae in your pool isn’t something you want but if you notice it, don’t panic. With some supplies, the right steps, and determination, you can keep it under control.
But remember, the best way to get rid of it is to not let it grow in the first place. So keep tabs on your pool’s maintenance and you should have nothing to worry about.
