How to Kill Black Algae in Your Pool

Swimming pools are great to have in your backyard in the summertime. A good, clean pool will give friends, neighbors, and neighborhood kids an excuse to come over all summer long! However, if you have a pool, you already know that it’s a lot of work and upkeep.

Unfortunately, if you aren’t proactive about keeping your pool clean, it can grow all sorts of unpleasant contaminants. Black algae are some of the worst! Black algae isn’t really an algae, but it can be even worse for your health than other types of pool algae if you swim in it or ingest it.

Due to black algae’s characteristic appearance, it’s relatively easy to spot if it grows in your pool. Cleaning it isn’t always the easiest task, but it’s a necessary step. In the remainder of this guide, we’ll teach you all about black algae, how to prevent it, and how to clean it up if you do find some.

What Is Black Algae?

Black algae are not true algae, but they are very similar. Instead of floating on your pool water’s surface, black algae prefer to root themselves into porous surfaces, such as the plaster or concrete lining of your pool. They’re single-celled organisms called cyanobacteria that can be black or dark blue-green.

Black algae are so tough to kill because they’re an encapsulated bacteria – they have a hard outer shell that makes them able to weather various harsh conditions. It’s difficult to kill them with chemicals alone; instead, it’s best to use a robust chemical sanitizer. A chemical sanitizer can break the shells, dislodge the algae from your pool walls, and kill them all at the same time.

Why are black algae dangerous to have in your pool? Well, cyanobacteria can also create cyanotoxins, which can make people and pets very sick. Just swimming with black algae can be very dangerous, but ingesting water infected with black algae is a severe health hazard that can leave you with lasting damage.

How to Recognize Black Algae

Because black algae don’t float on the top of your pool like some algae types, you shouldn’t be looking for it there. Instead, look for patches of “black mold” on the sides of your pool, under pool covers, and on any other porous toys or surfaces.

Don’t think that just because your pool is chlorinated, you’re entirely safe, either. Because of the encapsulated nature of black algae, they can survive in chlorinated conditions. If black algae are a persistent problem in your area, you can invest in a pool cleaner with a formula that works against black algae, but you’ll most likely still need to pair that with a good scrubbing.

Fortunately, black algae are relatively easy to recognize. Like we said earlier, if it looks like there’s “black mold” on the sides of your pool, then you have a problem. However, keep in mind that dirt and age can also mimic black algae’s appearance in your pool. Make sure to clean away any dirty patches and mineral stains to help prevent this issue from appearing.

If you suspect that you have a patch of black algae in your pool, don’t risk your (or anyone’s) health by going in! While black algae usually only cause stomach upset if ingested, its effects can go as far as permanent liver damage or neurological disease if you’re unlucky.

How to Kill Black Algae: Step-By-Step

Killing black algae in your pool can take some time. You’ll likely need to disinfect every affected surface while scrubbing everything thoroughly to dislodge the algae and break its heads. While some people prefer to drain their pool to make this step more manageable, it’s not required.

Make sure to check for any affected surfaces during this step. Check places like inside pumps and filters, behind and underneath ladders, inside pool lights, and deep inside cracks and crevices.

It’s a good idea to remove and sanitize the filter during this step, too; either soak it in a weak bleach-water solution to clean it or replace it entirely if you have an especially persistent infestation. Soak any pool accessories also, like skimmer baskets and vacuums.

If you have any pool noodles, throw them away. Their porous texture makes them hard to clean and provides an ideal spot for black algae spores to attach themselves to.

The next thing to do is acquire an algaecide for use in your pool. Even though black algae is not a true algae, most algaecides are formulated to work on black algae. Consider investing in one specially formulated for black algae if you have an unusually severe or recurring infestation. Always use the algaecide according to the manufacturer’s directions.

If you’re opposed to using an algaecide on your pool, some other chemical alternatives work well. Consider trying some of the following:

Once the algaecide has done its job, you’ll want to follow it up with a powerful chlorine “shock” to your pool (three times the usual dose or more). Sprinkle granular chlorine onto any places where you saw black algae forming and thoroughly rub it into these places with a stiff brush. Scrub those black spots with a stiff brush every day for a few days until they disappear.

It’s a good idea to shock your pool regularly to keep the black algae at bay. Some pool owners do this as often as once per week with great success. However, recoating or replastering your pool’s surface is a nearly foolproof solution to an unfixable black algae problem if all else fails.

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Preventing Black Algae Infections

As you may have realized, the most efficient way to take care of black algae infestations is to prevent them from ever happening in the first place! While you can’t always prevent the spread of the black algae bacteria, if you keep your pool at set parameters that discourage black algae growth, you can save yourself enormous trouble.

The first thing you can do to help prevent black algae formation is to discourage black algae spores from ever reaching your pool. When your pool is not in use, cover it thoroughly to prevent airborne spores from landing in your pool.

Never swim in your pool with swimsuits that were just in a lake or the ocean! Black algae spores are plentiful in nature, and they often hitch a ride on swimsuits and find their way into your pool. Make sure to wash and sanitize any swimsuits thoroughly before wearing them into your pool (it’s a good idea to politely ask guests to do the same, too).

Additionally, keep in mind that pets that have been in contact with natural water sources can also act as carriers. For this reason, you should consider not allowing your pets into your pool at all.

Finally, the more you stay on top of your pool’s chemistry, the less likely you will have a black algae infestation break out. As with any organism, the more regulated your pool’s aquatic environment is, such as pH, alkalinity, and chlorine levels, the less likely outside bodies are to thrive there. A robust and functional filter that you run often can reduce risks too.

As we mentioned before, shocking your pool regularly is a great strategy to keep your pool healthy. While black algae can tolerate intense chlorine levels, a highly-chlorinated pool is still better for discouraging infestations in the first place.

Questions? Let me know and I’ll be happy to help.

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