How to Kill Green Pool Algae in a Salt Water Pool

If they're operated properly, salt water pools are less likely to be vulnerable to algae growth than a traditionally chlorinated pool. That's because the chlorine generator that adds algae-killing chlorine to a salt water pool's water provides a constant supply of chlorine, and inadequate maintenance is less likely to give algae a chance to grow. Salt water pools aren't immune to algae growth, however, and the process of killing algae in a salt water pool is much the same as in a traditionally chlorinated pool.

Identifying Green Algae

Green algae is the type of aquatic plant that most commonly afflicts swimming pools. It can range in color from dark green to a much lighter yellow-green, and it often floats freely, turning the water a cloudy green color. It clings to surfaces as well and may coat the sides of the pool, equipment, ladders and steps.


The first step in getting rid of algae in the pool is to determine the starting conditions of the pool water. Test the water to determine the levels of free chlorine and cyanuric acid chlorine stabilizer in the water, as well as the water's pH level. Adjust the pH as necessary to reach a level between 7.2 and 7.5. Thoroughly brush all surfaces in the pool to remove as much algae as possible, as well.

Shock the Pool

To begin killing the algae, add enough supplemental chlorine to the pool to significantly raise the level of free chlorine, a process called shocking. This means adding liquid chlorine or other supplemental chemicals because the pool's chlorine generator isn't able to raise the chlorine level as high as it needs to be to kill the algae.

The target free chlorine level for this part of the process depends on the level of cyanuric acid in the water. A low level of cyanuric acid will result in too much free chlorine being lost to degradation from sunlight, but a high level will reduce the effectiveness of chlorine for killing algae, requiring you to add even more free chlorine to compensate. If the cyanuric acid level is 70 parts per million, add enough chlorine to raise the free chlorine level to 28 parts per million. If the cyanuric acid level is 80 parts per million, aim for a free chlorine level of 31 parts per million.

Test the pool's free chlorine level at least twice a day, and add more chlorine to maintain the target level. Brush the pool once a day, and clean the filter as necessary as it fills with dead algae.

Keep up the process until a test shows that the pool's combined chlorine level is no higher than 0.5 parts per million, the free chlorine level is not dropping overnight, and the water is free of visible algae. When these conditions are met, stop the treatment and allow the pool's free chlorine level to drop back to a normal range.

Algae Prevention

The most effective way to prevent algae growth is to maintain the proper level of free chlorine in the pool, which should be about 2 parts per million. Free chlorine levels may drop suddenly after heavy rains or heavy use of the pool, so be sure that the pool's chlorine generator is maintaining an adequate chlorine level in these situations.

Maintaining a proper pH level is important, too. Chlorine is much less effective at killing algae when the water's pH is high; at a pH level of 7.2, chlorine is 65 percent effective at killing pathogens, but at a pH of 8.0, its effectiveness falls off to 20 percent. Saltwater pools are particularly vulnerable to problems with elevated pH levels; a study conducted by the National Pool Industry Research Center suggests that the pH level of the water in a saltwater pool can creep up significantly in a matter of days. Check the water's pH regularly and adjust as necessary to keep the level between 7.4 and 7.6.