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Algae are microscopic plants which can transform your pool water from clear blue to a green, yellow black swamp in as little as 24 hours. Algae spores are introduced into the pool through the air, from rain water, and from swimmers walking across the grass and then jumping into the pool. Algae blooms are common after heavy rain showers and when the pool water temperature is high.

Algae feed off of nutrients that are present in a swimming pool or spa and are tiny plants that bloom and grow if a sufficient level of free chlorine is not maintained. Below are descriptions of the three most common algae problems in swimming pools.

Green algae are the most common algae in swimming pools and it floats in the water and coats or sticks to pool surfaces. Left unchecked green algae will very quickly turn the pool water pea green.

Green algae are very susceptible to chemical treatments. Hyper-chlorination using 10 to 20 ppm chlorine in the evening is a solid treatment method. Keep the filter running and brush the pool walls and bottom just after the application of chlorine to break-up the adhesion of the green algae to the pool surfaces. 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 green algae.

Mustard algae, settles on pool surfaces and walls and causes a slimy yellow film.

Mustard algae are much more resistant to chemical treatment and clings more tightly to pool walls than green algae. Adjust the pH level to 7.4 to 7.6 and hyper-chlorinate as for green algae then brush diligently with increased pressure as breaking down the adhesion of yellow algae is much more difficult. Follow up with vacuuming the pool, check chlorine and hyper-chlorinate again if necessary. Mustard algae will generally return unless treated with a special mustard algaecide or a copper based algaecide. Algaecides should always be added in the morning to treat algae in daylight since that is its most active period.

Black algae appears in buds or clumps attached to tile grout, corners, steps and pool surfaces.

Black algae are the most difficult algae to treat and are very difficult to get rid of. It can be controlled to some extent by frequent hyper-chlorination and diligent brushing with a stiff brush with high pressure applied. 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 as needed. Black algae can usually be controlled with the use of strong algaecides and maintenance of relatively high free chlorine residual are essential, but complete removal of black algae may require draining and cleaning the pool.

Black algae blooms are a problem best avoided. Maintaining proper water quality and frequent brushing of pool walls will deprive algae of the opportunity to get started and adding in borate to your pool water will prevent the nutrients that algae feed off of from being in the water.