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. Here are the steps you can take before draining your pool.

​​1. Clean or Replace the Filter

It’s a good idea to remove and sanitize the filter during this step. 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.

2. Test the Water

Test your pool water’s alkalinity, pH, and sanitizer. Know what the optimal levels are and aim to meet those. Adjust the levels accordingly before shocking your pool. Getting your water levels right will help you decide how much pool shock to use.

3. Brush the Pool

You’ll need the shock to be able to do its job so by brushing the algae off of the surfaces of the pool, the algae will be released into the pool where the shock will be able to kill the majority of it.

Make sure to get every possible crack to remove the algae from sticking onto anything. It’s a good idea to use a stronger, stainless still brush for this instead of your everyday pool brush.

You’ll want to focus some special attention on the areas of your pool where the algae didn’t come off. Use a stone, knife, or small wire brush to scrub these areas.

You can also try using a chlorine tablet, which will start to kill the bacteria while you scrub, offering double effectiveness.

After scrubbing your pool and the hard-to-reach places, brush your pool once more.

4. Quadruple Shock the Pool

You’ll want to follow up your brushing with a powerful chlorine shock to your pool, and a big one at that – four times the usual dose recommended by the manufacturer on the pool shock you use. You can use our pool volume calculator to figure out just exactly how much shock you need to use.

Pro tip – place all your pool equipment and accessories in the shallow end of the pool so they can get sanitized during the shocking process.

Run your pool pump for at least 24 hours so the shock can be evenly distributed and do an effective job at killing the bacteria.

After you’ve run the pump, brush the pool, yet again.

5. Clean the Filter

After running the filter to disperse the shock, your filter previously cleaned filter will have collected more algae. The good news is, this time, the algae was dying instead of living bacteria. Backwash or rinse your filter with a filter cleaner instead of replacing it this time.

6. Shock Again If Needed

If you see any remaining black algae in your pool after taking these steps, shock your pool again using a 2x dose of the manufacturer’s recommendations.

Run the pump for 24 hours if you can, or 8 to 12 hours at minimum, and brush the pool again. Brushing is important because you don’t want to risk the black algae coming back if there is any left.

7. Test the Pool Water

Once you take all the steps, test the pool water and ensure the chemical levels are accurate. Your pool should not have any more black algae but keep an eye out for any you may have missing or it returning. If even the smallest amount was left behind, it will start to reproduce and grow back. Give your pool a normal shock after brushing any algae off.

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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