Black algae, a common misnomer for a highly resistant organism, represents one of the most challenging pool maintenance problems. This persistent growth appears as dark, stubborn spots that adhere tenaciously to pool surfaces, signaling an aggressive infestation. Eliminating this growth requires a comprehensive protocol, combining aggressive physical removal with targeted chemical treatments. This guide provides a focused strategy for eradicating black algae and implementing preventative measures to maintain a clear and healthy swimming environment.
Identifying the Foe
Black algae is not a true algae but is instead a form of single-celled, photosynthetic bacteria known as cyanobacteria. Unlike common green algae, which float freely or cling loosely to surfaces, this cyanobacterium embeds itself deeply into porous materials like plaster, grout, and concrete. The organism’s difficulty to kill stems from its unique protective structure.
The cyanobacteria form a multi-layered, waxy biofilm that acts like a shield, making it highly resistant to typical chlorine concentrations. This thick, outer layer prevents sanitizers from penetrating the cell structure. Furthermore, the organism develops deep root structures that anchor it firmly into the microscopic crevices of the pool surface, meaning surface brushing alone is often insufficient for complete removal.
Step-by-Step Eradication Protocol
The treatment process must begin with preparation, ensuring the water chemistry is balanced to optimize the sanitizer’s effectiveness. Test the water and adjust the pH to a range of 7.4 to 7.6 and the alkalinity to between 80 and 120 ppm, as high pH reduces chlorine’s sanitizing power. Clean or backwash the pool filter before beginning treatment, as it is likely harboring spores that can cause re-infection.
The next step involves aggressive mechanical removal, which is non-negotiable for breaking the protective coating. Use a stiff-bristled steel brush for plaster or concrete pools, or a firm nylon brush for vinyl or fiberglass surfaces, to vigorously scrub every visible spot. The goal is to physically strip away the waxy layer and expose the underlying cell to the chemical treatment that follows.
Immediately following the scrubbing, the pool must be aggressively super-chlorinated. This requires a much higher dose than standard shocking, typically using a triple or quadruple shock dose of granular calcium hypochlorite, equating to about 3 to 4 pounds per 10,000 gallons of water. Apply the shock at dusk, as sunlight quickly degrades chlorine, and broadcast it directly into the deep end for maximum effect.
Maintain the high chlorine level for several days, running the pump continuously for 24 to 48 hours to ensure circulation. Continue to brush the affected spots at least once or twice daily during this period to continually expose the deeper cells to the concentrated chlorine. Once the spots have changed color from black to a grayish or white residue, they are dead and can be vacuumed to waste to prevent the spores from re-entering the water.
Specialized Chemical Treatments
Once the bulk of the black algae has been eliminated by the super-chlorination, specialized chemical treatments are required to target any remaining, deeply rooted cells. The most common and effective option is a quality algaecide specifically designed for black algae, often containing copper compounds. Copper algaecides are highly effective against this cyanobacteria, but check the pool’s current copper level before application, as excessive amounts can lead to staining on pool surfaces.
Concentrated chlorine or specialized tablets can be used for spot treatment on stubborn areas. A common method involves rubbing a broken chlorine tablet directly onto the remaining black spots to deliver a highly concentrated dose that penetrates the root structure. This technique is typically reserved for plaster or concrete surfaces due to the risk of damaging vinyl liners.
It is crucial to chemically clean or replace the filter media once the treatment is complete to eliminate trapped spores. For sand or D.E. filters, a thorough backwash followed by a filter cleaner is necessary, while cartridge filters should be removed and soaked in a chemical solution. This final step prevents residual spores from reintroducing the infestation.
Long-Term Prevention Strategies
Preventing the recurrence of black algae relies on establishing consistent, proactive maintenance habits. The foundation of prevention is routine water testing and the strict maintenance of sanitizer levels. Keep the free chlorine concentration between 1 and 3 parts per million (ppm) at all times to continuously kill any spores that enter the water.
Proper circulation and filtration are vital in preventing the growth of stagnant spots where the cyanobacteria thrive. Run the pool pump for a sufficient duration daily, typically 8 to 12 hours, to ensure all pool water is filtered and chemically treated. Regularly backwash or clean the filter to maintain optimal flow and debris removal.
Preventative brushing, even when the pool appears clean, should be a scheduled part of the weekly maintenance routine. Focusing on corners, steps, and shaded areas helps to disrupt nascent colonies before they can form their protective biofilm. All equipment, including pool toys, floats, and maintenance tools, should be rinsed and sanitized, especially after use in natural bodies of water, to avoid introducing new spores.