Black Mold and Algae in Your Pool: How to Kill It

What you had initially believed was flecks of dirt in your pool has since revealed itself to be something more permanent. You suspect black mold or algae, neither of which is great for your pool. How do you remove black mold and algae from a swimming pool?

To remove black mold and algae from your pool, use an algaecide and shock the pool. Be sure to disinfect all pool surfaces and check your pool pump to ensure it’s circulating water efficiently. Clean the pool filter as well.

In this guide to black mold and algae in pools, you’ll learn where these unwanted substances emanate from, if it’s okay to continue using your pool in their presence, removal methods, and prevention tips.

Let’s begin.

how to get rid of black mold in a pool

What Is Black Mold?

Black mold is technically called Stachybotrys chartarum, a microfungus variety that’s sometimes called toxic black mold.

Known as a common mold source, black mold doesn’t exclusively develop on surfaces in a midnight inky hue. It can also sometimes appear greenish.

The most frequent targets of black mold are paper products, wood, and cotton, but the spores can develop on other surfaces such as those around the house.

The best environment to encourage black mold to thrive is one in which the humidity is consistent and bountiful, there’s no sunlight, and the nitrogen levels are low.

Black mold doesn’t compete with other types of mold but still propagates the most healthily when no other molds are around.

Black mold will send its spores into the ambient air upon being disturbed. The most spore distribution will occur when black mold is wet.

Considering that you have black mold growing in or around your swimming pool, the mold is quite regularly wet.

Are Black Mold and Black Algae the Same?

Algae, just as mold, can appear on swimming pool surfaces in that same chilling dark hue.

Although algae are usually green, they can be black as well. However, that doesn’t make black mold and black algae the same.

Mold, black or otherwise, is a fungus. Algae are an aquatic plant group.

Let’s circle back around and talk about black algae for a moment. These, like other algae, grow in sizable groups.

Single-celled, chlorophyll-containing organisms stand out from green algae because black algae contain unique compounds.

It’s these compounds that reduce the green coloration that algae usually proudly display.

The compounds benefit black algae in more ways than a color change. The compounds will safeguard black algae to a degree so that it might be chlorine-resistant.

Compared to treating green algae in a swimming pool, you’ll have a harder time removing black algae.

Is Black Mold Harmful in your pool?

Knowing what you do about both black algae and black mold, you may have more questions still.

For example, is being in the presence of black mold hazardous to your health and the health of your family?

Well, black mold is called toxic mold, and that’s for a reason. According to Medical News Today, black mold produces mycotoxins that are then released into the air.

Prolonged exposure to black mold, such as living in a building with mold, could cause mold poisoning, which is also known as mycotoxicosis.

Mycotoxicosis can cause symptoms like nosebleeds, memory loss, headaches, mood changes, and body pain.

However, Medical News Today notes that these symptoms are rarely reported and thus might not solely be tied to mycotoxicosis.

Regardless, we’ve discussed on this blog that mold exposure can exacerbate the symptoms of those with asthma and/or mold allergies in your household.

Breathing in mold spores indoors can also reduce the quality of even healthy lungs.

In an outdoor environment such as a swimming pool, enough fresh air should be present that the above symptoms–if they are indeed tied to mycotoxicosis­–would be less likely to manifest but not impossible.

Is It Okay to Swim in a Pool with Black Algae?

Perhaps you’re dealing with primarily black algae in your swimming pool. Is it okay to let the issue go until the algae grow more significantly?

After all, it could be the middle of summer when a problem with black algae occurs. You might have a pool party scheduled that you’d prefer not to cancel.

Could a little bit of black algae hurt, or even a lot of algae?

Technically, the presence of the black algae will not cause an illness in any swimmers.

However, black algae can attract organisms that might harbor E. coli as well as other bacteria that could indeed cause illness.

Even if there was no risk of swimming in a pool that contains black algae, the presence of the algae is simply unappealing.

Once your friends and family realize that it’s not dirt lingering on the sides or bottom of the pool, they wouldn’t be in a swimming mood anyway.

The best course of action, if you have black algae in your swimming pool, is to remediate it, not ignore it. It will not away on its own.

How to Recognize Black Mold in Your Pool

Identifying black mold in your swimming pool can prove challenging, especially if the mold has formed deep within the water’s depths. The following pointers will make identification easier.

It’s Blue-Greenish or Pure Black, Not Brown

Dirt, even when wet, is typically some shade of brown. Black mold might be a midnight hue, or, as mentioned earlier, it can take on a blueish or greenish tint.

It’s Stuck Well on the Pool

With a little bit of elbow grease, dirt will lift from the pool liner and float to the top of the water, making the water cloudy in the process.

Black mold does not behave the same way. The mold grows in groups that feature raised heads. The clusters don’t turn the water cloudy and don’t disperse easily either.

Cleaning the Pool Alone Does Not Remove the Mold

When you go through the regular process of cleaning your pool, all dirt should be dislodged and removed. Your pool should also be spotless by the time you’re finished.

Black mold can remain if all you’re doing is scrubbing the pool walls or cleaning the bottom of the pool. The mold is sometimes even impervious to algae brushes.  

Why Does Black Mold/Algae Grow in Pools?

If you’ve owned a swimming pool for several years but this is the first you’ve seen black algae, you can be very concerned about its presence.

Here are some causes of black algae in swimming pools.

The Pool Is Too Warm

If your backyard lacks any overhead protection from trees, then the sun beats down on the water day in and day out.

While this makes for fantastic swimming conditions since the water never feels chilled, the warmth and brightness of the pool allow black algae to develop and thrive.

Lack of Filtration and Circulation

Your swimming pool uses a pool filter to catch contaminants in the pool and prevent them from circulating in the pool water.

A pool filter doesn’t stay clean forever. No matter which type of filter you have, you need to keep the media clean, or the filter can no longer do its job.

The pool pump is another integral part of your swimming pool that circulates water. If the pump is old or cannot keep up with the water volume in your pool, then the circulation will lag as well.

Black mold loves still, stagnant water, and will quickly dominate.

High pH Levels

As part of keeping your pool chemistry balanced, you should test the water’s pH regularly.

If you’ve forgotten to do that lately, then the levels could have risen past a comfortable point.

The high pH is quite favorable to black algae, which will spread rampantly.

How to Kill Black Algae in Your Pool

You’re aware now that black algae are harmful to keep in your pool, but how exactly do you remove them? Per the intro, here are the steps to follow.

Use an Algaecide

Algaecide is a chemical formulated to be safe for swimming pools as well as other water features.

Pouring algaecide into the water will kill the current algae lingering in the depths of your pool as well as prevent future algal growth.

While it varies based on the manufacturer, you should need only one bottle to treat up to 15,000 gallons of water.

Clicking the link below will take you to the Amazon store so you can check out a highly recommended Algaecide product that can be used in both swimming pools and hot tubs.

Clearwater Algaecide Algae Remover for Swimming PoolsOpens in a new tab. (UK)

Shock the Pool

Now is a good time to shock your swimming pool as well. Shocking the pool means increasing the free chlorine levels by adding more chlorine to the pool.

You should regularly shock the pool anyway at least once per season. Shocking will kill bacteria and algae so you can finally feel confident about inviting people to your swimming pool again.

Below are links to the Amazon store so you can check out some highly rater chlorine products.

Bluewater Chlorine Granules 5kgOpens in a new tab. (UK)

Clean Algae from the Surface

The algae in your swimming pool should be gone by now, but that does nothing to remove the surface-level algae.

To get rid of that, you should scrub the pool surfaces where the black algae remain. You can use a nylon brush, an algae brush, or even a pumice stone to take care of this job.

While you should put some elbow grease into cleaning, don’t be so forceful that you scrape your deck or damage your pool liner.

How Long Does It Take for Black Algae Killer to Work?

If you plan on treating black pool algae first but then still going ahead with your pool party, you want the algae killer to work immediately.

Precisely how long will you wait for the product to go into effect?

The answer can vary by product, but between applying algaecide and shocking the pool, you should expect the conditions to become clearer in as little as 24 hours and up to 72 hours.

Keep in mind that you will still have to scrape off surface-level algae, which will take more time (but not much).

Will Chlorine Get Rid of Black Mold in a Pool?

Every swimming pool owner has gallon jugs of chlorine handy, as the chlorine keeps the pool germ-free.

Regularly chlorinating your pool might not be enough to prevent black algae, though.

You’ll recall from earlier that black algae are moderately chlorine-resistant, especially if we’re talking about regular chlorine levels.

You’d have to shock the pool and perhaps even triple-shock it to make a difference in the number of black algae in the pool.

How to Get Rid of Black Mold on a Pool Liner

Has black mold sprouted up on your pool liner and you’re not sure how to remove it?

As we’ve discussed, using a brush or a scrubber to gently remove the mold from the liner is the best course of action.

Apply firm pressure yet keep your movements consistent so you don’t rip the liner.

Never use a pumice stone on your pool liner, as it will easily lead to tears. For other pool surfaces such as tile, plastic, concrete, or gunite, pumice is appropriate.

How to Prevent Black Mold and Algae from Coming Back

You’ve treated your swimming pool’s black mold and algae problem for now, but you’re concerned about recurring issues.

The following pointers will keep your pool free and clear for making summertime memories.

Check the Pool Pump and Filter

As we discussed earlier in this guide, when the swimming pool pump or filter underperform or malfunction, this is to the detriment of the cleanliness of your pool.

Keep the filter clean and check that both the filter and pump are working at least several times throughout the season. If you have to replace either of these components, don’t delay.

Shock Your Pool More Often

If you’ve had a recurring problem with black mold or algae in your pool, then it’s worth your while to shock it more than once per season.

Shocking doesn’t hurt the pool, and while you should advise friends and family members to stay out of the pool for 24 hours after shocking, that’s a short delay.

Test the Water Chemistry

You’ll recall that a high swimming pool pH is one of the causes of black algae in swimming pools. Thus, every two weeks, make it a habit to test your pool water chemistry.

Below are a few links to Amazon so you can check out some of our recommended pool and hot tub pH-testing products.

EASYTEST 7-Way Pool Test StripsOpens in a new tab. (US)

BOSIKE 3 in 1 Water Hot Tub, Swimming Pool & Spa Test StripsOpens in a new tab. (UK)


Black mold and algae in a swimming pool can ruin summertime fun, as neither is particularly safe to keep in the pool.

Now that you’ve pinpointed where your problem is coming from and you know how to treat it, you can safely enjoy the pool all season long.

Chris Walker

Chris Walker has struggled for several years with mold after buying his own property. After finding the solutions to several issues around his home, he decided to create this site in order to answer as many questions about mold and mildew as possible to help others dealing with the same problems.

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