Pools that have turned green due to algae overgrowth must be scrubbed with pool brushes before shock is added. Both the walls and floor must be scrubbed to dislodge the algae from their surfaces. Any algae remaining on the pool water's surface should be removed with a skimmer. The filter needs to be run for 12 to 24 hours to catch some of the loose algae, then back-washed to remove dead algae from the top of the filter media, according to the Swimming Pool Water Problem Solving Guide.
Pools turn green when the water's pH balance is out of whack. A pH balance that is too high, for example, can cause a growth of algae, while pools with low pH levels and water that is still green could be the result of a high copper level. Add a few chlorine tablets to pool water to lower the pH level or shock the pool with an un-stabilized chlorine, such as calcium hypochlorite, or an algicide. Shock doses are generally between 10ppm and 25ppm, depending on how severe the problem is.
Pools that have green water following a shock treatment may have dirty filters. Tree pollen that covers filters is broken down by the shock and eventually pushed back into the pool, resulting in a green tint to the water. This can be remedied by removing the filters from the pool and cleaning them with a water hose. Baking soda can be sprinkled into the pool's water to clear the it, which should occur in a day or two following treatment.
Green pool water is easily avoided with regular maintenance, such as using pH test strips in your pool a few times a week to monitor the pH balance and regularly vacuuming the pool to keep it free of bugs, debris and algae. Pool brushes may be used to periodically scrub the pool to prevent algae from growing. Regular skimming of the pool to remove leaves, dead bugs, debris and floating pieces of algae is also important.