What are the main causes of Algae in swimming pools?

What are the main causes of Algae in swimming pools?

Swimming Pool Algae and Algaecides

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Causes of Algae in Swimming Pools

There are many contributing factors that can lead to algae growth in your swimming pool, but the most significant is not having enough sanitizer – usually chlorine – coupled with poor water circulation from not running the pool pump long enough. Other common influencing factors that allow algae spores to grow include:

Neglecting to shock your pool, leading to higher levels of chloramines (chlorine that has done its job and is now inactive)

Improper pH and total alkalinity in pool water

Insufficient doses of stabilizer, creating a low chlorine level in the pool

Too much phosphate in pool water

Neglecting to use preventative doses of algaecide

Not running the pool filter long enough

An overall irregular or nonexistent pool maintenance program

A pool finish that is very porous

Weather conditions that are hospitable to algae, such as high humidity and heat

The three most common types of algae in swimming pools are green, black, and mustard algae. Here’s how to recognize and treat each one.

Green Algae

Green algae is a frequent visitor to residential swimming pools. It can vary in severity from adding a slightly green tint to the color of your pool water to as harsh as dark green fuzzy growths all over the walls and floor. In some cases, pool water is so heavily tinted with dark green algae that pool owners can’t see the bottom of their pools.

To treat green algae growth, shock your pool and use an all-in-one algaecide. Read label directions or call a pool supply company with your pool’s water capacity for recommended dosage and treatment instructions. After treating your pool for green algae, check the water balance – you may need to add an alkalinity increaser.

Black Algae

Black algae is not the most common type of algae found in swimming pools, but unfortunately if you do have it, it’s the most difficult type of algae to remove. The severity of algae can be as minor as a few small black spots to entirely covering the surface of the pool.

Black algae usually grows in clusters, creating large blotchy areas. It is difficult to kill because its roots grow deep into the pool’s surface and it has a tough outer coating, which makes it difficult for chemicals to attack.

To treat black algae, you will need to shock your pool and use an algaecide specially formulated to kill black algae. You will also need to use a metal control product. Be sure to adhere to a strict pool maintenance program that includes the weekly application of an algaecide to avoid a recurrence.

Yellow / Mustard Algae

Yellow or mustard algae is particularly common in swimming pools in the southern part of the US. It is most often seen as patches of dark yellow dust that covers various parts of the pool walls and floor. Mustard algae usually starts in the corners of the pool, near the steps, behind the ladder, and in other areas where the water circulation is poor.

Unless yellow algae gets into the cracks, pits, or crevices of the pool surface, it will brush off easily. Follow brushing with pool shock, then a treatment dose of algaecide formulated to treat yellow / mustard algae.

As always, follow label directions and pool chemical safety guidelines when treating algae in your pool. And remember, using algaecide weekly in a preventative dose will keep your pool clear and sparkling.


This eHow article on how to remove pool algae contains a photo illustration of thick green algae. However, PoolGear Plus advocates shocking your pool before adding algaecide, instead of the other way around as this article advises.