Why Is My Pool Water Green? How to Get Rid of Pool Algae

Algae are the most common cause of green pool water. You can get rid of pool algae and green water with good cleaning and maintenance.

The oxidation of copper in the water — from pool equipment, some pool chemicals or even the water itself — can give water a green tint. You can often treat this condition with metal-removing pool chemicals known as sequestrants.

Black algae will attach itself to the porous surfaces of your pool, like the concrete or masonry lining. It will look like spots on the pool surface. Ridding your pool of black algae requires chemical sanitization. 

Pollen can turn water a yellowish green. Filtering and skimming can help, but pollen is often too small for these steps to be completely effective. If pollen counts are high, you may want to use a pool cover when you’re not using your pool.

Algae are considered part of the plant kingdom. They’re photosynthetic organisms that use the sun, water and carbon dioxide to make their food. Algae cling to the walls, floor or steps of a pool or float in the water. They can grow quickly and multiply, so it’s best to treat algae as soon as possible, ideally at first sight. Pool algae generally aren’t harmful. However, harmful bacteria, like E. coli, parasites or salmonella, may exist in it.

Pool algae can be caused by poor filtration, out-of-balance water, low or inconsistent chlorine levels, or poor water circulation.

Green, dark green, yellowish green and blue-green algae are the most common. Green algae are slimy and are the leading cause of green pool water.

Black algae, which look like mold, can also form in a pool; however, black algae are bacteria. Black algae don’t float in the water. They cling in patches to rough, textured surfaces, like concrete, where they can penetrate the surface or grow roots into tile grout and cracks.

Other algae include yellow algae, which are a muddy yellow or brownish color, and pink algae, which, like black algae, are bacteria.

Keeping your pool algae-free requires regular maintenance. Below are a few things to consider that will help keep algae at bay.

• Clean the filter, pump and skimmer on a regular basis.

• Check the water balance on a consistent basis.

• Use an algaecide regularly. When used properly, algaecides can prevent algae growth.

• For normal water cleaning, make sure the filtration system is operating properly and circulates the water as it should.

• Shock the pool weekly. Shocking helps reduce cloudy water, which could be an indicator of early stage algae growth. Pools are generally shocked with concentrated forms of chlorine like calcium hypochlorite or dichloride. Calcium hypochlorite is a granular chlorine oxidizer that adds chlorine and calcium to water. It’s dissolved prior to adding to pool water. Dichloride also comes in granular form as well as tablet form. It also adds chlorine to the water as well as cyanuric acid. Shocking should be done prior to using an algaecide. If used simultaneously, they'll cancel each other out.

• Brush the pool regularly, including the steps, walls and floor. The type of pool brush you use depends on the pool material. A wire brush head works well on concrete or plaster pool surfaces but can damage acrylic, fiberglass, vinyl-lined or painted surfaces.

• Regularly vacuum the pool. A pool vacuum uses a skimmer to suction up debris from the pool.

It’s better to prevent algae than to later try to get rid of it. However, sometimes algae can creep up on you, developing in a matter of hours or days. The best way to combat algae is to address the issue immediately.

To remove green algae from your pool follow these steps:

• Test and balance your pool chemical levels using a pool test kit.

• Add pool shock to boost any residual chlorine in the pool, making sure to follow the label directions carefully.

• Use a pool brush to briskly scrub all surfaces where algae has developed, such as pool walls and steps.

• Vacuum or backwash to remove any remaining dead algae.

• Apply the proper algaecide according to label directions.

• Let the water circulate for 24 hours, then brush the pool surfaces again.

• Vacuum or backwash once more to remove any algae that may remain.