Swimming pool algae

Swimming pool algae, the 2nd most common pool water problem

Algae is a common problem in swimming pools.   What is it; what are the different types & how is it successfully  prevented and treated. First of all, algae is a single-celled plant  form. Some are aquatic (live in water), some are not.  Algae utilizes the process of photosynthesis to manufacture its own food. Algae comes in very wide variety of colors and forms making it adaptable to almost any environment.  

Although some forms are virulent, most are harmless and pose no more than a nuisance to  most pool owners... at least when treated early!

Due to algae's microscopic size, it  takes literally millions of these plants to accumulate to be noticed by the naked eye!  By that time it may be too late and very costly to correct. Prevention of algae blooms is the best solution.  

Let's look at the 3 most prevalent forms of swimming pool algae that we deal with: Green, Black & Mustard.

I'll also tell you how to best prevent & treat these algae problems at the bottom of this page.

A typical algae bloom in an above ground pool. Otherwise well taken care of but, 2 weeks of vacay...
A typical algae bloom in an above ground pool. Otherwise well taken care of but, 2 weeks of vacay...

Green Algae

Green algae, the most common & easy to treat

The most common form of algae that we deal with in swimming pools is "green" algae.  Green algae (varies in color from  blue-green to yellow-green to dark-green) can be free floating in the water (turning the water a hazy-green) or can be wall-clinging (patches of green).  

Wall-clinging varieties range in severity from small patches on pool walls and  bottoms to virtually covering the entire pool surface. Green algae has the ability to clog filters and may even cause surface damage if left untreated.  Green algae can be treated fairly simply and quickly with a proper, aggressive shock treatment & addition of a quality algicide.

A relative of regular green algae is "small-celled green algae" (SCGA). The  difference is seen in these areas: 1. The water remains relatively clear.  Many treat the problem (without proper analysis) as a copper or mineral problem,  however the metal chelants will show no effect.  2. When treating with chlorine, chlorine seems to "disappear".  SCGA is very resistant to even  high levels of chlorine (up to and over 10 ppm). 

Other mid-summer types of green algae noticed is "green spots" all around the pool, especially in shady areas. The water is almost always "very clear".  The water can have a "stinging" sensation.  This is normal green algae, typically brought about by lack of homeowner care;  i.e. not following a weekly maintenance routine such as the Once-a-Week 3 Step  program combined with very low pH (under 6.0) and very low Total Alkalinity (under 30 ppm).

Don't allow your pool to get like this... really
Don't allow your pool to get like this... really

Black algae

Fairly common, more difficult to treat, very preventable

"Black Algae" (actually  blue-green algae) forms in cracks and crevices on pool surfaces, especially plaster finishes. We normally find black pool algae growing in, but not limited to, shady areas of the pool. Black algae is more typically found in  concrete or plaster finished pools; it is very uncommon to find it in vinyl liner pools. It is known for a heavy slime layer and "skeletal growths" that make it impervious to normal chlorine levels, as well as its roots that can penetrate the pool surface finish.  As shown in the photograph, the water remains  relatively clear, however, almost all customers notice a high chlorine demand (use much more chlorine than normal). 

Since the end of 2009, we (and our  customers) have found that using AquaFinesse Pool Water Care Tablets dramatically reduces the probability of Black Algae because it removes the biofilm that creates the base or the foundation for the Black Algae. Regular, continued use of AquaFinesse Pool Water  Care Tablets is a great preventive measure against Black Algae.

Prior to and during treatment, the algae MUST be thoroughly brushed in order to "break open" the protective slime layer.  Failure to do this critical step will prevent the treatment from working; you have to get to the root of this form of algae. Pool chemicals  alone will NOT treat black algae.

BioGuard and its family of potent swimming pool algicides not only prevent algae growth, but are useful in treating algae blooms.

Algae with dark root
Algae with dark root

Mustard Algae

Mustard Algae, the most misdiagnosed algae of all

Mustard algae is a  chlorine-resistant form of green algae (yellow-green to brown in color)  typically found in sunbelt areas.  It often resembles dirt or sand on the bottom  or sides of a pool. When trying to distinguish between mustard algae or dirt, follow this common sense rule of thumb: if it feels gritty it's dirt; if it has slimy feel it's mustard algae. In our local market area (Fairfield county CT), the number of TRUE Mustard Algae cases that we treat can be counted on ONE HAND in most seasons. 


Mustard Algae has certain characteristics: It can be brushed away very easily, but returns quickly to the same location.  Keep in mind that the "algae" may be returning to the same place due to a dead spot in the pool.  Mustard Algae will also "climb" the pool walls; it will NOT simply stay on the bottom (this is where the confusion between dirt/dust and true Mustard Algae is realized). 

Although it usually creates a large Chlorine demand, it has been known to survive in high levels of Chlorine.  Mustard algae can also survive outside of the pool in toys, brushes, bathing suits, etc. It is extremely important to remove mustard algae growth  from equipment (including the back of underwater lights & ladders) and bathing suits to avoid cross or recontamination of other pools. Pool equipment can be left in the pool during product application or cleaned separately with a mild  cleaning solution. Rinse thoroughly if equipment is used in a SoftSwim® pool.  Bathing suits should be washed with detergent as directed on garment label.  

Mustard algae... ick!
Mustard algae... ick!

Treatment & Prevention

Easier to prevent than treat...

Algae Treatment:   

  1. Have the pool water properly analyzed. 
  2. BALANCE THE POOL WATER. Algae loves water with a higher pH.  
  3. Pools treated with chlorine or bromine should  aggressively shock with chlorine (BioGuard Burn Out 3®  or Smart Shock®) and use a good quality algicide such as Back Up 2®, Algae All 60®,  Banish® or better yet, BioGuard Algae Complete.  
  4. If possible, vacuum heavy accumulations of algae directly to waste especially if you have a sand filter. Perform this task PRIOR to chemical treatment.
  5. Pools treated with SoftSwim or other Biguanides  should top up their Bactericide level "B", add a double dose of algicide "A",  and add a double dosage of shock or "C".  

Algae Prevention:

  1. Follow the 5 Pillars of Pool Care. Sanitize, Oxidize, Prevent algae on a weekly basis coupled with good water balance.
  2. Run the filter 8 to 12 hours every day that the pool is open.
  3. Further prevent algae with BioGuard Optimizer Plus as part of the 3 Foundations of Pool Care. Borates are excellent in preventing algae and you'll have easier to maintain & silkier feeling water.