Algae is the most common pool water problem only because it is the most visibly obvious one. Algae in itself is not dangerous — in fact, it is the main ingredient in many health supplements and tablets. Algae brings danger in that it converts sunlight into food, releasing wastes that become the feeding grounds for unwanted and harmful bacteria and other micro-organisms.


There are well over 20,000 species of algae, only a few of which are encountered in swimming pool water. Algae is almost constantly entering the pool, and as soon as the sanitizer level drops too low, the algae begins to take hold and multiply. It takes as little as a few hours on a warm sunny day for your sparkling pool to develop an algae problem. The algae “spores” found in the air are from algae that have dried out (at a nearby stream, from a neighbor’s pool, etc.) and become airborne with the breeze.

Types of Pool Algae

Algae in swimming pools is often referred to by its color:

Green Algae

By far the most common and relatively easy to treat

Yellow/ Mustard Algae

Troublesome to treat but also relatively susceptible to treatment

Black Algae

Can be very difficult to get rid of, especially in plastered pools

Pink Algae

Not an algae at all, it is a fungus called Paecilomyces lilacinus that causes slimy white, pink or grey colonies

Preventing Pool Algae

Maintaining proper water balance and minimum sanitizer levels will ensure that any algae spores entering the pool water cannot take hold. Most pools use chlorine, which should be kept between 1.0 – 3.0 ppm.

Destroying Pool Algae

Once algae has been allowed to bloom, it is quite tiresome to destroy. The common steps to get rid of an algae problem are:

  • Shock treat the pool. If you are using chlorine, aim for at least 10ppm free chlorine (10 times the recommended minimum level).
  • Circulate the water round the clock, if possible.
  • Brush the pool and vacuum.
  • Add a commercial algaecide prepared for the algae you have (green, mustard or black) according to the instructions.
  • Repeat brushing and vacuuming daily if possible. Add more chlorine if the level falls below 5ppm.
  • Clean or backwash your filter regularly.

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