Black Algae: A Complete Guide to the Science

Most people dread spotting black algae in their pools. Are you one of them? If yes, this article will discuss how to ensure that the nightmare doesn’t turn into a reality any time soon.

For people lucky enough not to have spotted them, that calls for celebration. However, you also have to learn how to prevent it as well. Otherwise, you will end up having to deal with it one of these fine days.

What if your pool already has black algae in it, but you have no idea about what it really is? It is high time you confirmed that to deal with them before they get more challenging. This article defines what black algae is to help you know what you are dealing with for your advantage.

Besides defining black algae, it also highlights ways of identifying their existence in your pool. That’s reasonable since you can’t deal with something you have no idea exists. What happens once you spot them?

Again, this piece will take you through a step-by-step guide on how to kill black algae. It is no secret that getting rid of it can be tricky. However, with the procedures, the process becomes easy and straightforward. 

Other than the disgusting look, are they harmful to the swimmers? There is only one way to find out the answers to all those questions. So, without further ado, let’s take a look at them.

What Are Algae?

Before diving into black algae, do you know what algae are in general? According to scientists, they are one-celled plants with over 20,000 varieties. Those you will commonly find in a swimming pool include pink, yellow, brown, green, and black algae. 

They thrive in areas where the circulation isn’t normal and optimum. Such places include corners, walls, and steps of your pool, among other obstacles. Some are fur-like and slimy. It is also important to note that algae grow in colonies.

Our focus today is on one type of algae, Black algae.

What Are Black Algae?

Just like any other algae, black algae are an organism, living and growing organism for that matter. It has chlorophyll that helps it make its own food through photosynthesis. It is also aquatic with the necessary adaptations that make it grow and thrive in water.

Am much as it is an alga, black algae are one of the true algae. One of the differences between black algae and the other algae is how it grows. The rest of the algae float on the water surface of a swimming pool. On the other hand, black algae find porous surfaces such as the concrete lining or the pool’s plaster and root themselves on them.

Why Are the Algae Black?

Since chlorophyll is a green coloring matter, why are these algae black despite having it? Let me reiterate that black algae have chlorophyll, which gives them a green color and helps it during photosynthesis.

Experts classify black algae among bacteria, specifically cyanobacteria. As the name suggests, it is cyan which is a combination of green and blue colors. Therefore, black algae aren’t actually black but somewhat blue-green.

So, why does it still appear black despite being blue-green and containing chlorophyll? It is a result of water-soluble pigments, which are also some of its components. Consequently, the combination of the water-soluble pigments and the algae’s blue-green pigment is why the algae are black.

Why Is It So Hard to Get Rid of Black Algae?

Are you doing all that’s in your power to eliminate black algae, but nothing seems to be working? If your answer is a resounding yes, you are not alone because many people struggle with it. Out of all the other algae, the black one is the hardest to eliminate.

Why is it hard to eliminate? First of all, unlike other algae that float on the water surface, they root themselves. Therefore, it becomes hard to remove the deeply-rooted algae firmly attached to the concrete or plaster.

How comes that not even levels of chlorine seem to be working? Algae know all too well the need to struggle to survive, and survival is for the best. It has an adaptive mechanism that allows it to withstand even chlorine.

It has a lot of layers, and that’s how it protects itself from massacre by chlorine. If chlorine isn’t an option, can you imagine how to get rid of it? We will be taking a look at it very soon.

Why Do Black Algae Grow in Your Pool?

There are various reasons why black algae are growing in your pool. Some are natural hence hard to prevent, whereas others are manmade thus preventable. Let’s examine why you are dealing with black algae in your pool as we speak or will do in the future.

●        Heavy Rains

Rain, due to its nature, has a way of introducing black algae to your pool. It sweeps them off the decks, trees, plants, and other surfaces and then drains them in your swimming pool. If you are lucky enough, your pool will have the proper chorine levels to deal with them.

Unfortunately, if the amount of rain is too much, it becomes hard for your pool to maintain its chemical balance. After all, there will be too much rainwater flowing into the pool. Under such circumstances, the chlorine levels become too low to deal with the algae.

With nothing to counterattack it, the black algae grow without disruptions. Within no time, your swimming pool has algae, and you have to deal with it. That’s probably why your pool has it despite proper sanitation and filtration.

●        Wind

Wind can also carry black algae spores and drop them in your pool. After all, they are light and can easily float in the air. It is a common occurrence when living near an ocean, lake, or other natural water sources.

●        People

When someone swims in a lake or ocean, black algae may attach itself to their bodies. Therefore, if they enter a pool without showering, the algae will land on the pool. Again, that marks the beginning of a black algae infestation of your pool.

●        Clothing Especially Bathing Suits

How they introduce black algae to your pool is similar to how people to it. Just like with people, black algae can also attach themselves to clothing and swimming costumes. If a person enters the pool with them, they will transfer algae to it.

●        Dogs

Dogs can carry black algae spores easily. The algae usually attach itself to the dog’s paws or fur. If the dog enters the pool or the wind blows it off to the pool or water washes it to the pool, having black algae in your pool will soon become the new normal.

●        The Pool Water

If your pool water is dirty and unbalanced, black algae will blossom excellently. That’s because the bacteria love dirty water and thrives excellently under those conditions. For the record, they like it even better when it is highly unbalanced and dirty.

●        Filter

The role of the pool filter is to keep your water clean and balanced. However, if it clogs or breaks down, that will be hard to achieve. Consequently, the conditions will be ideal for the rapid growth of black algae.

Did you know that you have to let your filter run for at least 8 hours a day? Its purpose is to ensure that the water circulates properly. Otherwise, the water will become dirty and unbalanced, which isn’t bad for the health and safety of your swimming pool.

●        Skimmer Basket

Equally important, black algae-like growing in areas with poor circulation. Do you think it is impossible to result in that when you have a clogged skimmer basket? As a matter of fact, the water becomes cloudy within no time, and that’s how algae start overgrowing in your swimming pool.

The bottom line is black algae thrive better in some conditions than in others. They appreciate pathetic chemical balance, poor filtration, and terrible circulation. Finally, you should also be on the lookout for black algae spores because they are equally dangerous.

How to Identify Black Algae Growing in Your Pool?

Did you know that not all dark spots you see in your pool are black algae? If that’s the case, how then do you identify your enemy so that you choose the best strategy of combating it? These are ways you can use to determine if your pool has black algae.

  • The first way is the color of the spots, which is either blue-green or black.
  • Besides the color, the spots have raised heads.
  • They also don’t float freely on the water surface of the pool.
  • On the contrary, they attach themselves to the surface with roots.
  • Black algae also thrive on porous, rough surfaces such as pool plaster, gunite, or concrete.
  • If you try brushing them off the wall with a pool brush or the algae brush, you will be surprised to learn how persistent they can be.
  • In the beginning, they could look like mold in big clumps or a bunch of tiny black dots. 
  • To be sure that you are dealing with black algae, scrape off a relatively big chunk of what you may think to be algae. Then, spread it on a white sheet of paper or a paper towel. If you spot a dark green smear, then that’s it. Buckle up because you are about to enter a match with black algae.

Facts You Need to Know When Identifying Black Algae

  • Don’t assume that black algae couldn’t grow simply because you have a clean pool with an excellent filtration system.
  • It is easy to confuse with mineral staining since the two have several things in common. To start with, they tend to make the surface of the pool black since algae grow on it, whereas mineral staining discolors it. Secondly, they are not easy to scrape off the surface.
  • They hardly survive on surfaces such as vinyl liners and fiberglass.
  • They also have roots.
  • Black algae come from natural water bodies such as oceans. So, wash your swimming costume thoroughly after swimming in the sea before diving into your swimming pool.

How to Prevent Black Algae from Growing in Your Pool?

Given how hard it is to get rid of black algae, the best strategy is to prevent it from growing in the first place. Wouldn’t that save you from the struggles of having to deal with this nuisance?

We have already looked at the causes of black algae in your swimming pool, and prevention is as simple as tackling them.

Let’s dive into them since prevention is better than cure, especially in this particular case.

●        Shocking Your Pool Often

Use a relatively heavy chlorine treatment using the right shock product weekly. It will kill all the things living in your swimming pool that shouldn’t be there. It is advisable since it kills the spores before they bloom into the stubborn black algae.

●        Clean and Sanitize Every Pool Equipment or Toy

Preventive measures are all about preventing spores and black algae from entering the pool. Cleaning the pool equipment and toys before use will ensure that they don’t introduce them. That’s because water will wash the spores off the equipment and toy.

●        Wash and Dry Your Bathing Suits

Spores often stick themselves on cloth, and your swimming suit is no exception. That’s why you should wash them after swimming in an ocean or lake. Preferably, use bleach to wash them so that they can attack the spores.

●        Run the Pool Filter And Pump Often

Black algae thrive when the water is dirty. On the other hand, the filter cleans the pool water by removing unnecessary particles. Therefore, since black algae and spores are unwanted, running your filter will eliminate them. Run it for 8 to 12 hours daily so as not to give the algae time to bloom.

●        Keep the Pool Chemistry Levels as They Should Be

Since unbalanced chemical pools is also a cause of black algae in the room, maintaining the correct levels will ensure that not even a single spore survives. Check if they are balanced or not often. If not, do the necessary to regulate them back to normal.

Keep in mind that the right levels of chlorine, pH, and alkaline can deal with the algae squarely. 

How to Kill Black Algae Growing in Your Pool?

Have you tried your best to prevent black algae from growing in your pool, but they have defied all odds? All is not lost because there are ways of eliminating them once you notice an infestation. Follow these steps to kill black algae in your pool successfully.

#1 Gather All the Necessary Things Needed for The Job

Having the right tools for the job will make its completion easy and successful. Therefore, ensure that you have these things before you start handling the black algae in your swimming pool. Keep in mind that the right quality and quantity are equally important.

  • Bleach: 2 gallons
  • Pool brush: sturdy plastic for fiberglass or vinyl floors and stainless steel for concrete and plaster
  • Algaecide: 3 doses on average
  • Shock treatments: 1 pound for every 10,000 gallons. Therefore, the larger your pool, the more shock treatments you will need. It is usually a number between 4 and 10 in most cases.
  • Pool water test kit
  • Other pool chemicals: chlorine, alkaline adjusters, and pH adjusters.

Once you have them, now it is time to proceed to the next step.

#2 Sanitizing Each Piece of Pool Equipment

These are things that often harbor black algae spores then bring them to your swimming pools. That’s why you need to sanitize everything that comes into contact with your pool surface or water. They include pool floats and swimming suits, and there are various ways of sanitizing the different pieces.

Use warm water and a high quantity of detergent to wash towels and bathing suits. To sanitize pool toys, floats, and noodles, bleach will be the most appropriate choice. Also, use bleach to sanitize pool equipment such as the pole, brush, and skimmer net.

However, you also have to sanitize them further using chlorine hence the need to dip them into the pool. Don’t forget to empty, sanitize and scrub the pool pump and skimmer baskets.

#3 Deep Cleaning Your Pool Filter

The pool filter is an excellent hideout for black algae, and once they camp there, removing them becomes even more challenging. Therefore, it is essential to deep clean the filter lest the black algae return after a short while.

To avoid watering down your hard worth after a short period, don’t forget the filter as you clean the other parts.

Follow these few steps:

  • Use the filters pressure valve to release the pressure in it
  • After that, open the pool filter
  • Remove its filter media, a grid, cartridge, or sand, depending on the type.
  • Thoroughly scrub the filter interior, and don’t forget to sanitize it too
  • Replace the filter media, cartridge, DE grids & powder or sand, with a new, clean one
  • Alternatively, clean the old medium and then return it
  • Testing the water

Maintaining the proper alkalinity and pH levels discourages the growth of black algae. Therefore, you should test your pool water to check whether it is right or wrong. Remember that algae thrive in unbalanced water.

What are the right pH and alkalinity levels? The pH should be between 7.4 and 7.6. On the other hand, the proper alkalinity ranges from 120 ppm to 150 ppm.

Once you realize that your pool has levels below or above normal, don’t hesitate to adjust accordingly. Otherwise, the unbalanced water will make your pool the breeding ground of black algae.

#4 Brushing the Black Algae Spots

Before you commence, don’t forget that it won’t be easy. That’s why you need to do the brushing aggressively. 

It is advisable to use algae brush instead of the traditional one for great results.

Equally important, choose wisely. If you own a vinyl or fiberglass poll, choose a hard-bristled PVC or nylon brush. On the other hand, go for a stainless-steel pool brush if working on plaster or concrete.

As you scrub black algae, be merciless and use as much pressure as possible. It is the only way black algae will loosen its grip on your walls. 

Using a brush may not work perfectly, which is when you introduce chlorine tablets. Cut one into two halves and use the abrasive side to scrub the tough black algae spots. Ensure that you put on gloves when using the tablets to scrub. 

#5 Triple-Shock the Pool

Did you know that the recommended amount of shock treatment for a pool is 1 pound of shock per 10,000 gallons of water? If yes, that’s good, but this is an exception. As the name suggests, you will use 2 pounds per 10,000 gallons to triple shock your pool.

Therefore, the amount of shock will depend on the size of your swimming pool.

#6 Adding Algaecide

The next step is adding the algaecide to avoid a comeback by destroying even the spores that could bloom later. You can add one or two algaecide treatments. After that, run the pool filter.

#7 Clean the Filter Again

This time around, you will quickly clean and not deep clean your filter like was the case in the previous step. The purpose of the second cleaning is to remove the black algae it could have captured in the course of scrubbing them off the pool surfaces.

#8 Running the Filter

Run the filter to initiate water circulation.

#9 Scrubbing Again

Give your pool a second scrub, but with the filter running this time. Then, start looking for spots and work on them too.

#10 Shock Your Pool

You should do this two or three days after the no. 11 step. Equally important, use the standard dose of 1 pound of shock treatment for every 10,000 gallons of water.

How to Choose the Right Algaecide?

One crucial requirement when dealing with black algae growing in your pool is an algaecide. Therefore, it is equally important to choose the best for excellent results.

However, identifying one might be a daunting task due to the many options available in the market.

Luckily, this buying guide will help you choose the proper algaecide easily. By answering these questions, you will only take home the best.

What Algae Is the Algaecide Meant to Deal With?

It is no secret that there are various algae out there. It is also important to note that there are also multiple algaecides available. Some work on green algae, while others work for black algae, etc. Therefore, choose wisely lest you take home an algaecide for green or yellow instead of black algae.

Does It Stain Your Pool Surface?

Various algaecide has different ingredients, and some of them can stain your pool surfaces. The most notorious ones in staining are the copper metallic algaecides. However, using them as manufacturers instruct you will avoid that altogether.

What’s the Cost to Pay?

Some algaecides are more expensive than others. Nevertheless, the cheap ones may not be economical if you will have to use them regularly. Therefore, evaluate the price and the value to choose the best.

Do Their Effects Last for Long?

Again, this is something that the ingredients in that particular algaecide will determine. The longer they stay active, the more seldom you will have to treat your pool. Some will last three months, while others can go up to 6 months. Therefore, choose the one that’s best for your schedule, budget, and pool surface.

How Safe Is It to Swim in The Pool After Applying Them?

Some algaecides are safe; hence alright to swim as soon as you use them. However, experts recommend that you wait for at least 10 to 15 minutes before swimming after adding algaecides.

Follow the guidelines from the manufacturers regarding how long you need to wait. Additionally, don’t overdose on the pool if you want to swim sooner. Don’t forget that a high concentration of copper algaecides can stain your hair with a green ting or a bluish color.

Some of The Best Algaecide Available in The Market

For the best algaecides worth every penny, here are some recommendations.

#1 BioGuard Algae All 60

This BioGuard algaecide will effectively combat all types of algae, including mustard, pink, yellow and black algae. The manufacturer packages it in such a way that you need not measure before using it. Besides pools, it is also suitable for spas and fountains. It neither stains the pool surfaces nor disrupts the pH levels of the pool water.

Equally important, it ensures that black algae don’t return. Whether you used chlorine or bromine in your pool, you have nothing to worry about when using this algaecide. After all, it works harmoniously with both of them to ensure that the pool is at its best. It is also easy to use; hence no need for an expert.


  • It works on various types of algae
  • It also doesn’t stain your pool surface


  • You will have to apply it weekly

#2 Rx Clear Swimming Pool Algaecide 60 Plus

The Rx Clear algaecide is also an idea for killing black algae. Besides killing black algae, it can work on yellow and green algae. The non-forming polymer doesn’t leave any residue.

You don’t have to wait for a long time to swim after using this algaecide. That’s because it works fine and is also safe to jump into the pool right after using it. You don’t need to use much for excellent results.

4 oz. will work perfectly for every 10,000 gallons of water. You should use it for your pool every fortnight.


  • It is safe to swim immediately after using it.
  • It is versatile hence handles yellow, green, and black algae altogether.


  • If you use an excessive amount of the algaecide, it will foam.

Frequently Asked Questions  

Are black algae harmful?

Despite the striking resemblance that black algae share with black mold, the latter is not harmful. However, it takes away all the beauty of the pool, making it look unkempt and unattractive. Equally important, it feels gross to swim in such a swimming pool.

Nevertheless, it can cause harm by attracting E. coli and insects that may harm you by making you sick. Therefore, don’t let it infest your pool, and when it does, do what’s necessary to eliminate it.

How do you balance the pH levels of your pool?

Soda ash (sodium carbonate) will increase the pH level if they are below normal. On the other hand, if they are above normal, use muriatic acid or sodium bisulfate to decrease it.

How do you balance the alkalinity level of your pool?

If the alkalinity level of your pool is higher than usual, what should you do? Add muriatic acid or sodium bisulfate (dry). However, if it is lower than usual, use baking soda or sodium carbonate to increase it.


The article has left no stone unturned regarding black algae. You cannot prevent or eliminate something that you don’t know. That’s why the article has explained what black algae are and why it is black. It has also discussed what causes it, how to prevent it and how to eliminate them when prevention fails.

Are black algae harmful? How do you tell for sure that it is what you are dealing with? Al those answers are now at your disposal. When buying for an algaecide, you will have an idea of how to settle for the best. Equally important, there are some recommendations to consider when purchasing one.

If your pool water test doesn’t show average values, how do you proceed? Balancing pH and alkalinity levels are now crystal clear. The bottom line is that dealing with black algae just got easier. The ball is now on your court, and you have the power to make your pool even better.

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