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How to Get Rid of Black Algae Spots in Your Pool

Rob is a pool-service industry professional with over 20 years of experience.

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What Is Black Algae?

Black algae is a thread-like growth that develops on rough surfaces in swimming pools. It appears as dark black spots on the walls and floor of the pool, and is slippery to the touch.

This type of algae forms in a layered structure, with the outermost layer protecting the lower layers. Black algae in pools is similar to that which can be found in between bathroom shower tiles and on silicone seams near the bathtub. It is grows very slowly, but is extremely hard to get rid of once it has developed completely. It is also chlorine resistant. This is why you will need to shock the pool and use a silver-based algaecide to get rid of a black algae plague.

With the information I provide below, I hope to show you that this type of infestation is not as hard to get rid of as most pool service experts would have you believe. The most difficult part is making sure you brush all the affected areas thoroughly.

Pool with black algae spots.

Pool with black algae spots.

The Cleaning Process

Before adding any chemicals, you will need to:

  1. Brush every black algae spot with a steel-bristle brush to remove the protective layer. Nylon brushes will not work. (Note: If you have a vinyl lined pool, do not use a wire brush, as this can tear the liner.)
  2. Shock or super-chlorinate the pool using granular chlorine, also known as calcium hypo-chloride. TriChlor shock, which can be found at many large retailers, may also be used.
  3. Raise the chlorine to at least 15 ppm (parts per million) for 24 hours.
  4. Add eight ounces of silver-based algaecide per 10,000 gallons of water immediately after shocking the pool. Add the solution through the skimmer.

After 24 hours the spots may not be completely eliminated. It will take a few days for the treatment to work. You should continue to brush the affected areas regularly. I recommend a quart of algaecide as a precaution, and for regular maintenance. Silver-based algaecides work best, and my personal favorite is Silvertrine. It looks black when you add it to the pool, but don't fret. This is normal.

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This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.

Questions & Answers

Question: I have a gunnite pool and 3 spots have developed. They are about 3 to 5 inches in diameter. Two are black and one is a greenish brown, a little like rust spots but not I don't think. The last black spot seemed to appear overnight, I can send photos if that would help. How should I get rid of them?

Answer: I'm not sure if photos can be submitted on here, if so, by all means send. Are the spots on the walls or on the bottom surface or steps? This makes a difference. Does any debris get in the pool? Here's why I ask these questions... If organic debris such as leaves, acorns, etc.. sit on the pool bottom surface, they can create organic stains. These can easily be removed by turning off the pump and sprinkling a little calcium hypochloride granular shock on top of the stains. They will disappear within seconds. That said, I'm just not 100% sure what type of stains they are without seeing it.