There isn't a swimming pool owner around who hasn't worried at least occasionally about the appearance of algae in the pool. Algae are microscopically small plant forms that can contaminate a swimming pool, turning it into a sickly looking mess literally overnight. Most forms of algae can be destroyed by chemical disinfection, most commonly by using chlorine in high amounts. Even bleach in strong enough concentrations can kill off algae in a swimming pool, though it needs to be added carefully.
Swimming pool algae are classified by their colors, with yellow, green and black types most commonly contaminating pools. Most forms of pool algae can be killed through superchlorination and then manual cleaning of pool surfaces. Since bleach contains chlorine, if enough bleach is introduced into a pool, algae contamination can be effectively eliminated. To kill off pool algae, chlorine levels in the pool will need to be raised very high, up to 25 parts per million or more.
Superchlorination of a swimming pool is also more familiarly known as pool shocking. In standard pool superchlorination, chlorine levels are raised from five to 10 times the recommended level of pool chlorine, which is usually 1 to 2 ppm. Raising a swimming pool's chlorine level to 10 ppm, though, won't be enough to kill off all algae contaminating a pool. Typically, 20 ppm of chlorine in a pool is necessary to kill off algae. To kill pool algae using bleach, you will need several gallons.
A gallon of standard household laundry bleach contains from 5.25 to 6 percent sodium hypochlorite chlorine. Chlorine is chlorine, and the chlorine in bleach can eliminate algae from a pool if bleach is added in the correct amounts. It takes about 1 gallon of standard bleach per 30,000 gallons of water to raise a swimming pool's chlorine to 2 ppm. To achieve a 20 ppm chlorine level in an algae-contaminated pool, you will need about 10 gallons of regular bleach to achieve superchlorination.
When using bleach to shock a pool strongly enough to kill algae, keep in mind that bleach will raise a pool's pH. A swimming pool's pH should be between 7 and 8, with 7.2 to 7.8 being desirable. Bleach has a pH that runs from 10 to 16, so you'll need to lower pool pH by adding muriatic acid according to direction. Also, bleach is light-sensitive so add it at dusk and allow the pool to sit overnight.
Whenever using bleach in a pool for disinfection or superchlorination to kill algae, use only regular unscented bleach. Scented bleaches as well as splashless types have other chemicals in them that can upset a swimming pool's delicate chemical balance. In cases where algae have taken over a swimming pool, it's usually necessary to superchlorinate and then scrub the pool clean at least twice. Algae contaminating a swimming pool are very hardy and it will take several days to thoroughly eliminate them using bleach.