Black algae is a living organism that grows on porous swimming pool surfaces (think concrete, gunite, and plaster), and it shows up in the form of black spots. It has a nasty habit of settling into corners, steps, and other hard-to-reach areas, and it flourishes in both sun and shade. It’s also the most aggressive form of swimming pool algae, and it’s the most difficult form of algae to get rid of. Simply shocking or adding algaecide to your swimming pool will not kill it. There are two reasons black algae is so challenging: First, it features deep roots that keep it firmly embedded in your pool surfaces. Second, it has a natural, protective layer that makes it hard for chemicals to get through. But if you see the first signs of black algae on your pool walls, don’t despair: Black algae might be difficult to treat, but it’s not impossible. With the right pool care routine, you can kick those black spots to the curb and keep them from coming back.

How to Treat Black Algae

Black algae is tough — and if you want to remove it, it’ll take more than a quick scrubdown. Here’s what you’ll need to do to get rid of it for good:

Scrub your pool

Use a high quality nylon brush and give the black algae a good scrubbing. Remember, you’ll need something tough and sturdy if you want to get past that built-in protective layer. And don’t be fooled by appearances. Even if the black spots disappear, there may be “roots” beneath the surface, which is why scrubbing alone won’t take care of the problem.

Use chlorine tablets

Break a tablet in half and rub it directly onto the surfaces affected by black algae. This will help kill the roots and prevent it from growing back.

Clean your filter

Black algae flourishes when swimming pool water is in less-than-pristine condition. Keeping your pool filter clean and in tip-top shape should be part of your regular pool maintenance routine.

Shock your pool

Use granular chlorine to shock your pool water and kill off bacteria. Use three times the amount you’d normally use (about three pounds per 10,000 gallons of water). Shock your pool in the evening and make sure to let your pool filter run for at least 24 hours after you add the shock treatment. Wait a few days and do a follow-up shock treatment (this time, though, you can cut it down to one pound per 10,000 gallons).

Repeat as needed. Don’t be discouraged if it takes more than one treatment to remove all of the black algae. Keep scrubbing and shock your pool as needed until the problem goes away.

How to Keep Black Algae From Coming Back

Want to make sure black algae doesn’t find its way back into your pool? The best way to prevent a return is to keep up a regular pool maintenance routine.

This includes testing your water regularly to make sure your pool chemistry is balanced. Use test strips to check alkalinity, pH, and chlorine levels, and make adjustments as needed. You should brush and vacuum your pool and run your pump regularly to remove dirt and debris. Continue to shock your pool once a week, and don’t forget to clean any pool accessories or toys, too!

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