Algae are single-celled organisms that photosynthesize and thrive in pool water when conditions favor growth. This proliferation occurs due to poor water circulation, insufficient sanitization, or an imbalance in the water’s pH chemistry. When algae spores, constantly introduced by wind and rain, find a nutrient-rich environment, they rapidly multiply, turning a clear pool cloudy and green. The goal of algae removal is to kill these organisms, remove the resulting debris, and restore the pool to a safe, clear state.
Diagnosing the Algae Infestation
Identifying the specific type of algae present is crucial, as different varieties require varied treatment. The most common type is green algae, which appears as a hazy tint or slimy patches clinging to the walls. Green algae is the easiest to eliminate with standard super-chlorination.
Yellow or mustard algae is a chlorine-resistant strain that often resembles yellowish-brown dust, typically gathering in shaded or low-circulation areas. This type often requires specialized algaecides due to its resilience to standard chlorine treatment.
The most difficult to treat is black algae, which is actually a dark blue-green bacteria. It forms colonies with a protective, slimy layer and root-like structures that embed into porous surfaces like plaster. Black algae appears as dark, raised spots and usually does not affect the water’s overall clarity, but it demands an aggressive, multi-step approach. Correct diagnosis determines the required chemical dosage and the intensity of physical scrubbing needed.
Physical Preparation and Initial Balancing
Before introducing high concentrations of sanitizer, all pool surfaces must be vigorously brushed to break up the protective layers algae form. Use a stiff-bristled brush, or a wire brush for concrete or plaster pools, to scrub the walls, floor, steps, and areas with poor circulation. This exposes the algae cells to the chemical treatment. This physical disruption is important for yellow and black algae, as failure to remove the outer layer impedes chlorine effectiveness.
After brushing, vacuum any large debris or heavy concentrations of algae from the pool floor. Ideally, use the filter’s “waste” setting to expel the material directly rather than sending it through the filter. Removing this organic matter prevents it from consuming chlorine during the chemical treatment phase. The final preparatory step involves adjusting the water chemistry, aiming for a pH level between 7.2 and 7.6, which optimizes the sanitizing power of chlorine.
Super-Chlorination and Algaecide Application
Super-chlorination, also known as shocking, involves raising the free chlorine concentration to kill the algae. Liquid chlorine (sodium hypochlorite) or granular chlorine (calcium hypochlorite) are commonly used. Application should be performed at dusk or night to minimize the sun’s ultraviolet rays from rapidly degrading the chlorine. The dosage is determined by the severity of the infestation, targeting 10 to 20 parts per million (ppm) of free chlorine.
For a heavily green pool, the dosage may need to be two to four times the standard shock treatment. This may require up to two gallons of 12.5% liquid chlorine or one pound of 75% calcium hypochlorite per 10,000 gallons of water to reach 20 ppm. Safety gear, including gloves and goggles, must be used during handling and application to prevent chemical burns or irritation. After the initial high-dose shock, a specialized algaecide, such as one designed for mustard or black algae, can be added to target any residual spores or strains that survived the chlorine.
Filtration and Debris Removal
Once the algae have been killed by super-chlorination, the water will turn a cloudy gray or white as the dead organic matter remains suspended. Clearing this cloudiness requires the filtration system to run continuously, often for 24 hours or more, to capture the microscopic particles. The filter will quickly become saturated with dead algae and debris. This necessitates frequent backwashing for sand and DE filters or cleaning for cartridge filters, indicated by the pressure gauge rising 8 to 10 psi above its clean reading.
If the water remains cloudy, a chemical aid can be used: a clarifier or a flocculant. A clarifier binds small particles into larger clumps that the filter can capture, requiring the pump to run continuously. A flocculant binds particles into heavy masses that sink rapidly to the pool floor, requiring the pump to be turned off for 8 to 12 hours. The debris settled by a flocculant must then be vacuumed and expelled “to waste,” bypassing the filter entirely to prevent clogging.
Ongoing Prevention Strategies
Maintaining consistent water circulation requires the pool pump to run a minimum of 8 to 12 hours daily to prevent stagnant areas where algae thrive. Regular physical cleaning, including brushing all pool surfaces once a week, dislodges spores before they can establish colonies. Brushing also allows the sanitizer to reach potential growth spots.
Weekly testing of the water chemistry ensures the free chlorine level remains between 1 to 3 ppm and the pH is stable between 7.2 and 7.8. Routine chemical testing and physical cleaning prevents the conditions that allow algae spores to bloom.