Pool Algae: Recognizing It, Clearing It and Keeping It Away

Applies to: In-ground and above-ground home pools with vinyl, fiberglass or plaster surfaces, using chlorine (including salt systems). Follow your product labels and equipment manuals.

How to recognize algae

Kern County Public Health describes algae as tiny plant growth that may feel slimy and can look green, brown or black. Signs include green, cloudy water and dark green or black spots on the pool surface.^[1]

Zodiac's pool-care page describes three common kinds:^[4]

• Green algae: the most widespread and the easiest to eliminate.^[4]
• Yellow or mustard algae: forms spots on the walls and floor.^[4]
• Black algae: the most stubborn and difficult to get rid of.^[4]

Check corners and fixtures too. Hayward notes algae can gather on walls and on ladders and slides.^[5]

Why algae grows

• Not enough chlorine, and skipped routine maintenance such as testing and shock treatment (Kern County).^[1]
• Disinfectant at the wrong level, or not enough filtration (Zodiac).^[4]
• High pH or incorrect water hardness, which Zodiac says reduces how well treatment products work.^[4]
• Warm water. Zodiac says algae thrive in it.^[4]
• Plant debris and biofilm left in a poorly cleaned pool (Zodiac).^[4]
• Plaster that is etched or cracked (Kern County).^[1]

Act early. Zodiac notes that the longer you wait, the more algae spreads and the longer treatment takes.^[4]

Safety first

Clearing algae: the general sequence

Kern County's public pool maintenance sheet lists superchlorinating, brushing spots with an algae brush and using an approved algaecide among its remedies for algae. The steps below combine those with manufacturer guidance. How much of any product to use, and when, comes from its label and your pool's volume, not from this guide.^[1]

1. Test chlorine and pH. CDC says a pH that is too high or too low makes chlorine less effective, and recommends pH 7.0 to 7.8 and at least 1 ppm chlorine in pools, or at least 2 ppm if you use cyanuric acid (stabilizer) or stabilized chlorine products.^[7][11]
2. Brush. Hayward's TigerShark guide says to brush the walls and chemically treat for algae, and Kern County lists brushing spots with an algae brush. Zodiac says brushing helps detach algae spores.^[1][4][12]
3. Superchlorinate (shock) by the label. Kern County lists superchlorinating as a remedy, and Hayward says to follow the manufacturer's directions for shock treatments.^[1][5]
4. Use an algaecide only if its label covers your situation. Kern County lists an approved algaecide among the remedies.^[1]
5. Vacuum up the brushed-off algae and sediment, as Hayward describes, and clean and maintain your filter according to its manufacturer.^[5]
6. Retest, and don't swim until the drain at the deep end is clearly visible.^[6][11]

Hold off on automatic cleaners during the worst of it. The Polaris 280 manual says not to use the cleaner during shock chlorination, and Hayward's TigerShark guide says that when pool condition is unacceptable for the cleaner, you should sweep walls, vacuum and balance the chemicals first.^[12][13]

Mind your surface. The Polaris 280 manual notes some patterned vinyl liners can be scratched or worn by a pool brush. Check your liner or pool builder's care instructions before scrubbing hard.^[13]

What algaecide labels mean

EPA lists algaecides as a type of pesticide, used to kill or slow the growth of algae. With limited exceptions, a pesticide must be registered with EPA before it can be legally sold or distributed in the United States.^[2][14]

EPA says pesticide labels are legally enforceable: "the label is the law." Every label carries a statement that using the product in a way inconsistent with its labeling violates federal law.^[3][15]

EPA notes that labels tell you how to use a product safely, first aid instructions, and phone numbers for help.^[16]

Preventing algae

• Keep sanitizer up. San Diego County says pool sanitizers kill bacteria and inhibit the growth of algae, and CDC notes pool chemicals can protect against algal growth.^[17][18]
• Test regularly. CDC recommends checking chlorine and pH at least twice a day.^[11]
• Brush walls and fixtures weekly and skim often, as Hayward recommends.^[5]
• Keep the filter clean and maintained according to its manufacturer.^[5]

A neglected green pool is also a mosquito problem. San Diego County says mosquitoes that can carry West Nile virus and other diseases breed in green backyard pools, in as little as a quarter inch of standing water. Its vector control program asks residents to report green pools they notice; check with your own county.^[17]

When to call a professional