Color-coded Tips for Treating Algae in the Swimming Pool

Algae in the swimming pool is an unwelcome sight, but one that usually can be dealt with effectively. The following “color-coded” tips can help you or your pool service professional identify and eliminate, or at least control, the most common types of algae. It is important to follow manufacturers’ directions for using and storing all pool chemicals.

Green Algae

Green algae usually appears when pool sanitizer levels are low or water circulation is poor; green algae turns pool water cloudy and murky. It is the easiest type of algae to remediate, but left unaddressed, green algae can worsen to the point of obscuring the floor and steps of the pool and potentially even a struggling bather, raising the drowning risk. Eradicate green algae by raising the chlorine level or adding an algaecide. Following treatment, it is important to run the filtration system continuously to clear the water by trapping the dead algae in the filter.

Avoid green algae by maintaining proper pool chemical levels. Additionally, avoid cross-contaminating your pool with green algae spores by thoroughly washing swimsuits and water toys before re-using them in the pool following a visit to a natural water body.

Black Algae

Black algae appears as black dots on pool plaster, especially where the plaster is pitted, etched, or where calcium deposits have developed. Poor water circulation in those areas helps protect black algae. In fact, the more deteriorated the plaster, the worse the potential problem.

Tackle black algae as soon as it appears by scrubbing the affected area with a stainless steel brush. The scrubbing action removes a protective coating that develops over black algae. Daily brushing is required until the algae spots disappear. In severe cases, it may only be possible to control black algae, and not to eliminate it completely. It is essential to keep scrubbing black algae spots as soon as they appear.

An additional recommended step is to treat affected surfaces with “trichlor” on horizontal pool surfaces and a copper-based broad spectrum algaecide on vertical pool surfaces. Broad spectrum chelated copper algaecide can also be used as a preventative measure. A word to the wise: Have your pool water analyzed for metal levels prior to adding any metallic algaecide, such as copper-based products. If metals are present in the pool water, adding a metallic algaecide could cause the pool water to become oversaturated with metals, which could lead to staining or water discoloration. It is also advisable to keep copper to a minimum from a human toxicity standpoint. Most drinking water municipalities keep copper levels below 1.3 ppm. The manufacturers of copper algaecides should be able to provide the “ppm added” from use of their product. Excess copper in swimming pools can cause blue-green coloration of the hair, nails and skin.1

Yellow/Mustard Algae

Yellow or mustard-colored algae is probably the most challenging type of algae to eradicate. First, the diagnosis may be difficult. Yellow algae in a blue pool may look green, but the pool is not likely to contain green algae unless the water is cloudy and murky. Yellow algae may also be mistaken for dirt or pollen. The diagnostic test is to see whether or not the yellow substance returns to the side walls of the pool after it is scrubbed. Dirt or pollen will drop to the floor of the pool, but yellow algae will reappear on the walls.

To further complicate matters, yellow algae can thrive in a well-balanced chemically treated pool. It can be introduced into your pool from lakes, ponds, on the wind or in the rain. It can also be transmitted through previously contaminated pool equipment, such as skimmers, leaf rakes or vacuum hoses and heads. Treat yellow algae with an algaecide or chlorine enhancer that specifically targets yellow algae.

Anti-algae Basics

To help avoid algae of all types, keep in mind the basics of chemical and physical pool maintenance.

CHEMICAL MAINTENANCE: Proper pool chemistry means free chlorine and pH are within target ranges. According to the US Centers for Disease Control and Prevention’s Model Aquatic Health Code, pool water free chlorine levels should be in the range of 1-4 ppm and the pH should be between 7.2 (minimum) and 7.8 (maximum). The ideal pH in a pool is 7.4 to 7.6.

PHYSICAL MAINTENANCE: Physical maintenance of the pool means keeping up with brushing and vacuuming the pool and cleaning skimmer and pump baskets. Algae love rough and deteriorating pool surfaces, so sanding and crack repair are helpful. Clean pool filters as the first step in battling algae. Operating pumps and filters for at least 8 to 10 hours per day throughout the summer keeps water moving, a deterrent to algal growth.

This article is based on “Tips and tricks for identifying and treating tough algae,” by Terry Arko in the March 2017 edition of The IPPSAN, a publication of the Independent Pool & Spa Service Association, Inc.

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1 Peterson, J. et al. (2006). Cupric keratosis: green seborrheic keratoses secondary to external copper exposure. Europe PMC 77(1): 39-41. Abstract online, available: