What Causes Black Algae in Pools [Unveiling the Culprits]

By Algal Web

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A pristine and inviting swimming pool is a source of joy and relaxation. However, the appearance of black algae can quickly turn this oasis into a source of frustration for pool owners. Black algae, a stubborn and persistent pool problem, can not only mar the aesthetics of the pool but also pose potential health risks. 

In this article, we will delve into the causes behind the formation of black algae in pools, unraveling the factors that contribute to its growth, and providing comprehensive insights into prevention and treatment.

Understanding Black Algae

Black algae, scientifically known as Phormidium tenue, are a species of cyanobacteria that frequently infest swimming pools. Unlike other algae varieties, black algae possess a unique structure that makes them particularly resistant to eradication efforts. 

They have a protective layer that shields them from chlorine and other common sanitizing agents, allowing them to firmly attach themselves to pool surfaces. Differentiating black algae from other pool issues is crucial for effective treatment. 

While green algae float freely in the water and can be relatively easy to remove, black algae penetrate the porous surfaces of the pool, making them difficult to eliminate. Moreover, the presence of black algae not only affects the visual appeal of the pool but can also pose potential health hazards to swimmers.

Common Causes of Black Algae Formation

  • Insufficient Chlorine Levels: Chlorine plays a pivotal role in preventing algae growth in pools. When chlorine levels are low, it creates an environment where black algae can flourish. Black algae have developed resistance to low chlorine concentrations, making it crucial to maintain adequate chlorine levels to discourage their growth.
  • Poor Circulation and Filtration: Proper water circulation and filtration are vital for maintaining a healthy pool environment. Inadequate circulation results in areas of stagnant water, which provide favorable conditions for black algae growth. Insufficient filtration fails to remove algae spores and organic matter effectively, providing nourishment for black algae to thrive.
  • Lack of Pool Maintenance: Neglected pool maintenance routines significantly contribute to the growth of black algae. Regular brushing of the pool walls and surfaces is essential to dislodge any algae spores and prevent them from establishing a foothold. Neglecting routine cleaning allows the accumulation of organic matter, such as leaves and debris, which act as nutrient sources for black algae.
  • Introduction of Contaminants: External sources of contaminants, such as rainwater, windblown debris, and even swimmers, can introduce algae spores into the pool. Algae spores are microscopic and can enter the pool unnoticed, where they find a suitable environment for growth. Swimmers themselves may unknowingly carry algae spores on their bodies or clothing, inadvertently introducing them into the water during pool usage.
  • Imbalanced Water Chemistry: Maintaining proper water chemistry is critical for a healthy pool environment. Imbalances in pH and alkalinity levels can create conditions favorable to black algae growth. Moreover, high levels of phosphates and nitrates in the water act as triggers for black algae, fueling their proliferation.

Preventive Measures

Image Credit: cleanup.expert

To prevent the formation of black algae in pools, proactive maintenance practices and preventive measures should be implemented:

  • Regular Pool Maintenance: Establishing consistent cleaning routines is vital. Regularly brush the pool walls and surfaces to dislodge any potential algae spores. Skim the water surface and remove debris to eliminate nutrient sources for black algae. Additionally, vacuum the pool regularly to remove any settled debris that may act as a breeding ground for algae.
  • Proper Water Chemistry: Regularly monitor and maintain appropriate chlorine levels to inhibit algae growth. Test the water frequently to ensure chlorine, pH, and alkalinity levels are within the recommended ranges. Adjust these levels as necessary to maintain a well-balanced pool environment.
  • Optimal Circulation and Filtration: Ensure proper water circulation by running the pool pump and adjusting the direction and intensity of jets or nozzles. This helps prevent stagnant areas in the pool where black algae can thrive. Regularly clean and maintain the pool filter to ensure optimal filtration, effectively removing debris and algae spores.
  • Minimizing Contaminant Introductions: Implement preventive measures to reduce the introduction of external contaminants. Install a pool cover when the pool is not in use to prevent leaves, insects, and other debris from entering. Maintain surrounding vegetation and ensure proper drainage to prevent rainwater runoff from carrying additional contaminants into the pool. Educate swimmers about the importance of showering before entering the pool to minimize the introduction of algae spores.

Treatment Options

In cases where black algae have already infested the pool, immediate and targeted treatment is necessary:

  • Recognizing Black Algae Infestation: Identifying black algae growth patterns is crucial for effective treatment. Black algae typically appear as black or dark green spots on pool surfaces. These spots often have a rough texture and resist brushing. Distinguishing black algae from other pool problems, such as dirt or stains, is essential for targeted treatment.
  • Targeted Cleaning and Treatment: Treating black algae requires a thorough and systematic approach. Begin by vigorously brushing the affected areas to break through the algae’s protective layer. Use a pool vacuum to remove dislodged algae debris from the water. Select appropriate algaecides and chemicals specifically designed to combat black algae, following the manufacturer’s instructions carefully. It may be necessary to repeat the treatment process multiple times for complete eradication.
  • Restoring Balance and Preventing Reoccurrence: After treatment, restore proper water chemistry by adjusting pH, alkalinity, and chlorine levels as needed. Regularly monitor the water and make necessary adjustments to maintain optimal balance. Implement the preventive measures discussed earlier to minimize the chances of black algae reinfestation.


Understanding the causes of black algae formation in pools is essential for effective prevention and treatment. By addressing factors such as insufficient chlorine levels, poor circulation, inadequate maintenance, contaminants, and imbalanced water chemistry, pool owners can significantly reduce the likelihood of black algae growth. 

Adopting regular maintenance practices, practicing proper water chemistry, and taking proactive measures to minimize contaminant introductions are key to maintaining an algae-free and enjoyable swimming pool environment. With diligence and care, pool owners can keep their pools sparkling clean and inviting for endless hours of swimming pleasure.

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