How to Remove and Prevent Green Algae in Swimming Pools

Use chlorine as an algaecide. When the pool water turns green or visible algae clusters appear, it means the chlorine level in the pool is too low. "Shocking" the pool by adding a large amount of chlorine is the most effective way to kill algae and restore the pool to a clean condition. This method usually works within 1-3 days but may take up to a week if the situation is severe.

• The methods listed below work faster but may not address underlying hygiene issues. These methods are also more expensive and may include unwanted side effects.

Scrub the walls and floor of the pool. Vigorously scrub to remove as much algae as possible. This step will help reduce the time needed to kill algae and eliminate existing growth. Pay special attention to steps, behind ladders, corners, and crevices where algae tend to accumulate.

Ensure the safety of pool chemicals. You are handling hazardous chemicals with this method. Always read the safety instructions on the product label before use. At a minimum, follow these safety standards for pool chemicals:

Adjust the pool's pH level. Use a pool pH test kit to measure the water's pH. If the pH is above 7.6—common during algae outbreaks—use a pH reducer (like sodium bisulfate) as directed on the product label. Aim for a pH between 7.2 and 7.6 to enhance chlorine effectiveness and reduce algae growth. Wait a few hours, then retest the water.

Select a shock chlorine product for the pool. Regular pool chlorine may not be the best choice for shocking. Ideally, use a liquid chlorine product specifically designed for pools, containing sodium hypochlorite, calcium hypochlorite, or lithium hypochlorite.

Add a large shock dose. Check the chlorine product label for shocking instructions. To kill algae, use double the recommended dose for normal shocking. Triple the dose if the water is very cloudy, or quadruple if you can't see the top step of the pool ladder. Pour the shock directly into the pool water while the filtration system is running.

Recheck the pool water the next day. After the pool filtration system has been running for 12-24 hours, you should inspect the pool again. Dead algae will turn white or gray, either floating in the water or settling at the bottom. Regardless of whether the algae are dead, you need to recheck the pH and chlorine levels.

• If the chlorine level is higher (2–5 ppm) but algae remain, maintain this level for a few more days.
• If the chlorine level has increased but remains below 2 ppm, you need to perform another shock treatment the following evening.
• If there is no significant change in chlorine levels, the pool may have too much cyanuric acid (exceeding 50 ppm). This condition results from using granular or tablet chlorine, which can 'lock' chlorine in an unusable form. The only way to address this is to repeat the shock treatment (sometimes multiple times) or partially drain the pool.
• Leaves or other debris in large quantities can also deplete chlorine. If the pool has been unused for a long time, it may take a week and multiple shock treatments to resolve the issue.

Scrub and inspect daily. Vigorously scrub to prevent new algae growth on the pool walls. Chlorine will eliminate algae within a few days. You need to check daily to ensure the chlorine and pH levels are acceptable.

• A well-maintained pool should meet the following standards: Free chlorine: 2-4 ppm, pH: 7.2 – 7.6, Alkalinity: 80 – 120 ppm, and Calcium hardness: 200 – 400 ppm. Minor deviations from these standards are normal, so slight variations are not a concern.

Vacuum dead algae. Once the pool water is no longer green, vacuum all the dead algae until the water clears. You can skip this step and let the filtration system handle it, but only if you have a high-capacity filter and are willing to wait several days.

• If you encounter difficulties removing the algae, you can add a flocculant or coagulant to clump the algae together. These products are available at pool supply stores but may not be worth purchasing for a home pool.

Clean the filter. If you have a D.E. (diatomaceous earth) filter, set it to backwash mode. If using a cartridge filter, remove and clean the cartridges with a high-pressure hose, followed by treatment with diluted muriatic acid or liquid chlorine if necessary. If the filter is not thoroughly cleaned, dead algae can clog it.

Enhance water circulation to tackle small algae spots. If only a few small algae clusters have formed and haven’t spread across the pool, there might be areas where water isn’t circulating properly. Check if the water jets are functioning correctly. They should spray at an angle that creates a spiral-like water flow.

Use a coagulant to gather algae. Flocculants or coagulants help clump algae together, making it easier to vacuum out live algae. This process might take a full day of effort, but the pool water will clear up once completed. It’s the fastest way to make the pool look clean, but it does not make the water safe for swimming. If algae can thrive, so can viruses and bacteria. After this, you can shock the pool to sanitize it, and avoid swimming until chlorine and pH levels return to normal.

Treat the pool with algaecide. Algaecides are effective at killing algae, but their use may not always be worth it due to potential side effects. Here are some factors to consider when using this method:

• Some algaecides aren’t strong enough to handle large algae blooms, especially black algae. Consult store staff for assistance or look for products with 30%+ active ingredients.
• Quaternary ammonia algaecides ("poly quats") are affordable but can cause foaming, which many find unpleasant.
• Copper-based algaecides are more effective but expensive. They can also stain pool surfaces.
• After using algaecide, wait at least 24 hours before adding other chemicals.

Maintain pool water chemistry. Algae won’t grow if you keep the pool water in optimal chemical condition. Regularly check free chlorine levels, pH, alkalinity, and cyanuric acid levels. The earlier you detect an issue, the easier it is to resolve.

• Ideally, test daily, especially for one or two weeks after an algae outbreak. Always test at least twice a week throughout the swimming season.

Using algaecide as a preventive measure. Algaecide works most effectively when applied in small doses weekly while the pool is in normal condition. This method eliminates algae before they have a chance to proliferate. Check the product label for specific instructions.

Eliminate phosphate compounds. Algae thrive on nutrients in the water, particularly phosphate compounds. A phosphate test kit is an inexpensive way to check for these chemicals in your pool. If present, use a commercial-grade phosphate remover available at pool supply stores. Run the automatic filter or use a manual vacuum to remove the phosphate remover within a day or two. Apply pool shock treatment once phosphate levels return to an appropriate level.