Identifying Pool Algae by Colors and Types and How to Destroy Them

What's that green, yellow, or black slime in your pool?

Green Algae Grunge before Cleaning Tiled Swimming Pool

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If you can think back to your science class you may recall that algae are one-celled plants. And if you happen to be one of those algae, you are not alone: there are more than 20,000 varieties.

In a swimming pool or spa, algae are those green, brown, yellow, black, or pinkish slime that resemble fur growing on the steps and in corners — places where circulation may not be optimum. The majority of swimming pools are exposed to several hours of sunlight per day, and it's sunlight that will speed up algae growth.

If you notice an increase in algae growth, run the pump more often. Also keep the water balanced, which means more frequent testing. It may seem obvious, but remove things from the pool, such as floats, inflatable toys, leaves, and grass. Make a real effort to keep it clean. The following algae colors are described and remedies are recommended in The Ultimate Guide to Pool Maintenance by Terry Tamminen.

Here's how to identify and get rid of certain types of pool algae.

  • 01 of 04

    Green Algae

    Green algae — chlorophyta — is a slimy substance that can be found on pool and spa surfaces. First signs of it appear in small clusters on pool steps or lurking in corners. It's at this stage that you should start to attack it — green algae can grow quickly in 24 hours or less.

    Brushing will remove green algae, but it won't destroy it. Superchlorination, aka shocking or shock treatment, will sanitize pool water that might be resistant to normal chlorination. Maintaining your pool regularly during swim season is key to staying on top of a green algae outbreak.

  • 02 of 04

    Yellow Algae

    Yellow algae also goes by the appetizing term "mustard algae," because of its brownish or muddy yellow color. While yellow algae (phaeophyta) doesn't spread as quickly as green algae, it is harder to destroy.

    Like green algae, yellow algae grows in the same fur or mold-like pattern. Unlike green algae, brushing will not do much to remove it, although it will remove the top layer of slime, which exposes the algae underneath. Superchlorination and regular maintenance will help kill mustard algae.

  • 03 of 04

    Black or Blue-Green Algae

    Black and blue-green algae are one and the same. Primarily found in lakes and ponds, it does find a home in unmaintained pools. According to author Tamminen, black algae is "the pool technician's worst nightmare." He continues, "at the first sign of black algae, you need to consider the pool or spa as a patient in critical condition."

    Why? Unlike green or yellow, black algae doesn't have that outer layer of slime, which acts as a protective barrier for the algae underneath. Black algae will penetrate deeply into hard surfaces, like plaster and concrete. It first appears as black spots and then proliferates. If you see signs of it, immediately use a stainless steel brush, which cracks the algae's shell and allows sanitizers to penetrate it.

    Blue-green algae were formerly known as cyanobacteria, a blue photosynthetic bacteria that live in water or damp soil.

  • 04 of 04

    Pink Algae

    A bacteria, pink algae appear as a reddish-orangeish slime, usually at a swimming pool's water line. It is also attracted to PVC surfaces in a pool, especially piping. It seems to keep company with white water mold, which is a fungus. Of all its colorful cousins, pink algae are the easiest to maintain. Simple brushing and regular sanitizing should keep it under control. If it has gotten out of hand, remove it, then shock it with a calcium hypochlorite shock.

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  1. Tamminen, Terry. The Ultimate Guide to Pool Maintenance. McGraw Hill Professional, 2007.