Algae in Your Pool Causes and Effective Removal

It’s the bane of every pool owner’s life: the green growth that appears in the still corners of the pool’s steps and quickly spreads to the skimmer box, the tile grouting, and various other surfaces. I’ve had pools where it never appears and others where it’s challenging to eradicate. Why, then, do some pools have visible algae that has grown out of control?

Algae exists in every pool as it occurs naturally in the debris that lands in the water, and its spores are blown in by the wind. It is best controlled using chlorine, but it also requires the chemical balance of the water and the pH level to be correct in order for the chlorine to be effective.

Because it’s such a common problem, the presence of algae in our pools is an issue I’d like to cover in this article. It’s something we should be able to tackle ourselves instead of relying on professional pool experts. That means understanding what the various types of algae are, why they are a problem in some pools and not in others, and what we need to do to remove them and keep them out of our pools.

Why Do Pools Have Algae And What Exactly Are They?

Algae are present just about everywhere and use all types of transport to migrate from one area to another. They float in the air, get carried by animals and waterbirds, and even by people who have been sailing or swimming in any body of water.

What Are Algae – Plants, Animals Or Neither?

Living things are classified into five kingdoms: animal, plant, fungi, protist, and monera. Algae belong to Kingdom Protista. They are organisms that possess photosynthetic pigments like chlorophyll but don’t possess true roots, stems, or leaves that characterize vascular plants. They may be unicellular or multicellular.

Most algae are aquatic, living in freshwater, seawater, and moist terrestrial environments. When there is sufficient light and nutrients, they can “bloom” into large colonies, which is what you see happening when your pool develops areas of green, yellow, or black algae.

Why Does Algae Grow In A Pool?

There are several reasons why the algae in your pool grow and bloom so that you have visible signs of them and water that has lost its sparkle and taken on a greenish tinge.

  • If the chlorine level is too low or is allowed to fluctuate.
  • If the pH level is too high (above 7.8), the water has become too alkaline.
  • Cyanuric acid, or pool stabilizer, prevents chlorine from breaking down too quickly, making it effective for longer in keeping the water clear. Too much, however, prevents the chlorine from working and encourages algae growth.
  • Phosphates in pool water come from garden fertilizer run-off, rotting leaves and vegetation in the water, certain pool chemicals, and the municipal water supply. Too high a level of phosphates is another reason for algae growth.

What Types Of Algae Are In Pool Water?

As I mentioned, many different types of algae are found in swimming pools, none of which are harmful but may provide the breeding ground for bacteria.

Green algae are the most common form of pool algae. They can float in the water or attach themselves to the surfaces of the pool, particularly areas where the water doesn’t circulate, like corners and shallow shelves.

Green algae are known scientifically as Chlorophyta, containing and colored by chlorophyll. It is found in various colors ranging from dark green to yellow and can proliferate in 24 hours, so I recommend removing it and treating it as soon as you spot the first signs.

Black algae can be blue-green, but whatever the color, it’s a lot more problematic than the other types of algae. It first appears as small black spots but grows quickly and penetrates deeply into the plaster or concrete surface of the pool and is very hard to eradicate. Leaving it for too long will result in permanent damage to your pool, apart from it being very unsightly.

Mustard algae is also called yellow algae, with the scientific name phaeophyta. It looks similar to sand on the pool floor, but while it doesn’t grow as quickly as green algae, simply brushing it off the surfaces and vacuuming it won’t remove it.

What Is The Best Way To Get Rid Of Pool Algae?

Let’s first look at how to remove algae from the pool and then work out how to prevent them from continuously returning.

How to Remove Green Algae

Green algae are the easiest of the three to remove. Begin by brushing down the walls, steps, and hard-to-get-at corners with a stiff wire brush to get the algae onto the pool floor. Once this is done, vacuum the pool thoroughly to remove the algae, and then clean your filtration system by backwashing. It might be necessary to repeat the last step a few times to make sure the algae are totally eliminated.

How To Remove Black Algae

“Black Algae” doesn’t affect the clarity of the water but forms in the cracks and irregularities of concrete and plaster surfaces, very often in the shaded areas of the pool. The problem in removing it is partly because of its location but also because it develops a slimy protective layer, which makes it resistant to chlorine.

The solution lies in a tough scrubbing action, which cracks the protective layer of slime. If this isn’t done, no amount of shock treatment will be effective. You may have to repeat the process of scrubbing followed by aggressive doses of shock before the black algae is eliminated.

How To Remove Mustard Algae

Mustard algae is also a chlorine-resistant form of algae and the most difficult to eradicate because it can survive outside the pool on, for example, bathing suits, pool equipment, toys, and floats that have been exposed to mustard algae in lakes, dams, or even your own pool. All these items will have to be disinfected to avoid re-infecting the pool.

The cleaning of the pool itself requires the same process as I’ve described for removing black algae, but mustard algae are even more stubborn and chlorine-resistant, so you may need a specialized algaecide specified for mustard algae.

All the methods I’ve outlined will work only if they are followed up by shocking the pool by drastically increasing the chlorine level to kill off algae and bacteria, and then, after 48 hours to allow the chlorine level to return to normal, check and adjust the pH level, cyanuric acid (stabilizer) level, and total alkalinity.

How To Prevent Algae From Returning

Keeping algae at bay in your pool means weekly shock treatment, regular brushing down of the walls and floor, and keeping the chemical balance constant by testing the water regularly and making the necessary adjustments.

Algae can’t be eliminated, but maintaining the chlorine levels is the best way to control their growth. This means monitoring all the chemical elements, ensuring the pool is always clean, and that the filtration system works to full capacity. An alternative to consider is converting your pool to saltwater, as the beauty of this system is that the chlorinator keeps the chlorine level constant without needing daily monitoring.


1. Why is my pool slimy?

If your pool is green and slimy, the algae has gotten out of control, and the only solution is the shock treatment we’ve mentioned. High chlorine dosage, followed by brushing down all the surfaces of the pool, and thorough cleaning of the filtration system should solve the problem, but the treatment needs to be repeated regularly.

2. Will pool clarifier remove algae?

Pool clarifiers are designed to clear cloudy water by binding small solid particles together so that they can be more easily vacuumed out. While they are a useful tool to keep the pool water sparkling clear and will remove algae that are floating in the water, they are not a substitute for your other pool chemicals and should be used in addition to your normal maintenance program.

3. How do I treat yellow algae?

As mentioned above, yellow algae is often called mustard algae and is a stubborn one to remove. Firstly, try and eliminate the source of the yellow algae by washing bathing trunks, pool equipment, toys, and floats that might have carried the spores from rivers, lakes, or the sea into your pool. Then follow the same procedure we’ve detailed for the removal of black algae.

4. Can one swim in a pool that has algae?

Algae are not dangerous in themselves, but they provide nutrition for bacteria and are a sign that your pool is not in a healthy condition. While you can swim in a pool with signs of algae growth, it would be best to treat the water as we’ve discussed to not only get rid of the algae but to ensure that the chemical balance has been restored and the water is sanitized and perfectly healthy to swim in.


It’s impossible to keep algae out of your pool, but with care and attention, it isn’t difficult to control their growth, ensuring that the water is crystal clear and the surfaces free of their visible presence. Once they start to multiply, I’ve detailed the best ways to remove them. The secret to controlling algae? Keep your chlorine levels constant, use shock treatment regularly, and vacuum the surfaces of your pool often.