Algae are tiny plants that bloom and grow in swimming pools if nutrients are present and a
sufficient level of free chlorine is not maintained. Below are descriptions of the three most
common algae problems in swimming pools.
Green Algae The most common algae in swimming pool floats in water and coats swimming
pool surfaces. Left unchecked green algae will very quickly turn the swimming pool water pea
Mustard Algae settles on swimming pool walls and causes a slimy yellow film.
Black Algae appears in "buds" or clumps attached to tile grout, corners, steps and swimming
Green Algae – is very susceptible to chemical treatment. Superchlorinate with 10 to 20 ppm
chlorine in the evening. Keep the filter running and brush the swimming pool walls and
bottom. Periodically check chlorine and maintain above 3 ppm until water clears. Using an
algaecide containing quaternary ammonia the next morning will help prevent the return of
Mustard Algae - is much more resistant to chemical treatment and clings more tightly to
swimming pool walls than green algae. Adjust pH and superchlorinate as for green algae then
brush diligently. Later vacuum the swimming pool, check chlorine and superchorinate again if
necessary. Mustard algae will generally return unless treated with a special mustard algaecide.
Algaecide should be added in the morning to treat algae in daylight - its most active period.
Black Algae - is very difficult to get rid of. It can be controlled to some extent by frequent
superchlorination and diligent brushing with a stiff brush. Spot treatments can be made by
turning off the recirculation pumps and pouring granular chlorine directly on recently brushed
spots. Trichlor tablets can also be rubbed on recently brushed areas to spot treat. Black algae
can usually be controlled with the use of strong algaecide and maintenance of relatively high
free chlorine residual, but complete removal of black algae may require draining and cleaning
the swimming pool.