If the water in your swimming pool is turning green, odds are you are dealing with the common problem of pool algae, which does not necessarily reflect sanitary conditions of the water. Even if the algae are forming slowly and are barely visible, the water inside your pool can still turn a slight shade of green. Since no one wants to swim in green water, take the time to stop the pool water from turning green and forming more algae.
Understand how to shock your pool water. This process should always be done at night. Dissolve each dose of shock in a bucket of clean water, then add to the water one at a time.
Double shock your pool water. This can help get rid of algae in the beginning stages just as your water is turning green. Shock can be purchased from a pool supply store or home improvement center. Normally, you need 1 lb. of shock for every 10,000 gallons of water, so to double shock the water, use twice that amount. You may need to shock a third and fourth time.
Add flocculant to the water if shocking doesn't work after a day or two. This chemical, also available at pool supply stores, will cause the algae to drop to the bottom of the pool.
Use a swimming pool vacuum cleaner to clean up the algae on the bottom. This could take a while but will get rid of the existing algae.