Start by vacuuming the pool as thoroughly as possible to suction and scrub away algae from the bottom. Brush the sides of the pool with a light-bristled brush and run the filter continuously until the water begins to clear. Circulation is important to cycling out the algae. After you vacuum, backwash your filter according to the manufacturer's guidelines to flush any collected dirt and algae from the filter system.
Super shock can be anywhere from 10 to 20 percent stronger than standard chlorine shock treatments. Add one dose of super shock to the water and let the filter run continuously. Routinely check the skimmer basket for circulation clogs and monitor the filter pressure gauge. A reading of eight to ten pounds per square inch above normal readings means the filter must be backwashed. Perform as necessary to ensure proper circulation from the filter. Shock the pool aggressively until some clearing occurs.
Pool supply stores sell algaecide that should be added after you shock the pool. While chlorine eliminates most forms of bacteria, an algaecide should be added to prevent new algae from forming while you are in the process of clearing the pool water. The algaecide should be applied directly to the pool water according to the manufacturer's recommended dosage. Prior to adding the algaecide, vacuum the pool and brush the sides again. Backwash if the pressure gauge indicates that it's necessary.
If You Fail, Try Again
Clearing a green swimming pool can be a real pain. It takes a lot of maintenance and often the brush, vacuuming, algaecide and shock treatments will have to be repeated. The algae has a habit of resettling towards the bottom of the pool as the water circulates. If you add enough shock to the pool it will eventually begin to clear up. The algaecide should also aid your efforts as well. Proper filter maintenance and chlorine routines are essential to preventing algae formation.