Algae are photosynthetic organisms, which means they use sunlight to create energy. Algae may be single-celled or multicellular. When algae is present in a pool, the water can look cloudy. There are three main types of algae found in swimming pools: green, yellow (also known as mustard algae) and black algae. Algae is not only unsightly but can also cause surfaces to be slippery.

Green algae, which is free-floating, is the most common. These organisms do not attach to anything, but float near the sides of the pool. Green algae flourishes if the pool’s filtering system is not working properly or if pH and chemical disinfectant levels are off. Lightning storms can also encourage green algae growth, as the electricity in the air reduces the free chlorine content in the water, while upping nitrogen – an important food for green algae.

Yellow or mustard algae does not have chlorophyll. Its long growing process makes the organism less common and difficult to treat.

Black algae can be quite aggressive, attaching itself to the sides and bottom of the pool. Despite its name, it may be dark blue or green. With a waxy coating, black algae is the most difficult to eradicate.

Good pool hygiene requires the prevention and removal of all types of algae. The best time to do this is at the warmest and sunniest time of the day, when algae is growing and most vulnerable. Algae growth can be stunted with filters that are in good working condition. Other treatments include algaecide, chlorine-based swimming pool shock and gentle scrubbing. Acid washing may be necessary for extreme cases. Pool ionization can help prevent algae from appearing at all.