The discovery of dark, stubborn spots on pool walls often signals an infestation of “black algae,” a term that misidentifies this challenging organism. This growth is not true algae but a highly resilient form of cyanobacteria. Its presence indicates a lapse in sanitation and requires an aggressive, multi-step approach to eliminate it completely. Treating this growth involves understanding its biological defenses and committing to rigorous chemical and physical methods.
Understanding Black Algae
The organisms commonly referred to as black algae are actually a species of cyanobacteria, which are photosynthetic bacteria. This distinction is important because the bacteria possess a layered defense mechanism highly resistant to standard pool treatments. The colonies form a thick, dark, gelatinous biofilm composed of polysaccharides and proteins that shields them against sanitizers like chlorine. This protective outer layer must be physically breached before any chemical treatment can be effective.
The cyanobacteria also embed themselves deeply into porous surfaces such as concrete, plaster, or grout, anchoring the colony firmly in place. This rooting mechanism and the external biofilm explain why simple shocking or routine brushing is often insufficient to eradicate the infestation.
Health Risks of Swimming
Swimming in a pool contaminated with black algae is advised against due to several potential health concerns. Cyanobacteria, especially when forming large colonies, can produce compounds known as cyanotoxins, which are harmful to human health. Exposure to these toxins can lead to adverse reactions.
Direct contact with the water can cause skin irritations, such as rashes, hives, and itching, or result in eye and throat irritation. If the contaminated water is accidentally swallowed, individuals may experience gastrointestinal issues like vomiting, diarrhea, and stomach pain. Children are at a higher risk because they often spend more time in the water and are more likely to ingest it.
Furthermore, the aggressive chemical treatments necessary to eliminate the cyanobacteria require the pool to be taken out of service. The water remains unsafe for swimming until chemical levels return to normal.
Step-by-Step Removal
Eliminating a black algae infestation requires an aggressive, sequential process that first targets the physical structure of the colonies. The initial and most vigorous step involves manually scrubbing the affected areas with a stiff-bristled brush, such as a wire brush for plaster pools. This action penetrates the protective biofilm and exposes the underlying bacteria. Following this scrubbing, chlorine tablets should be broken and rubbed directly onto the remaining spots to deliver a high concentration of sanitizer where it is needed most.
The pool must then be subjected to a hyperchlorination process, often called “triple” or “quadruple” shocking, using a high dose of calcium hypochlorite or an equivalent shock product. This massive increase in sanitizer concentration is necessary to kill the exposed cyanobacteria and spores suspended in the water. A specialized black algae algaecide should also be added after the shock treatment to target any remaining cells that survived the initial chemical assault.
Throughout this treatment period, the pool pump must run continuously for at least 24 to 48 hours to ensure the highly chlorinated water circulates effectively. After the spots have visibly disappeared, the pool surface must be brushed again. The debris should be vacuumed to waste to physically remove the dead organisms and spores from the pool environment. Finally, the filter system, including the cartridges or backwash lines, must be thoroughly cleaned or chemically treated to eliminate any trapped cyanobacteria that could cause re-infestation.
Prevention of Future Growth
Preventing the re-establishment of black algae involves maintaining a consistent and rigorous pool maintenance schedule. The most important preventative step is ensuring the pool water chemistry is always balanced, especially the sanitizer and pH levels. Free chlorine levels should be consistently maintained within the recommended range of 1.0 to 3.0 parts per million (ppm), and the pH should be kept between 7.2 and 7.6.
Regular, weekly brushing of all pool surfaces is another powerful preventative measure, as it disrupts the initial formation of biofilms and removes microscopic spores before they can anchor. This routine brushing should specifically target corners, steps, and shady areas where water circulation is naturally lower.
Proper water circulation is also fundamental, requiring the pump and filtration system to run for a sufficient period each day, typically between eight and twelve hours. This prevents stagnant water zones.