There are only three things that will clear up a green pool:
Your filter system, your pool chemicals and you !

Unfortunately, there is no magic way to turn a green or black pool clear overnight ! It usually takes a lot of time, effort and money to clear up a typical "swampy" green pool. It is certainly best if you never let your pool water get this dirty, however most pool owners experience a "green pool problem" from time to time.

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Although it is not easy work, there is a proven method of clearing up a green, black or very dirty pool in the shortest time possible. Follow the instructions below for the best results.

1. Remove most of the large debris from the pool floor with a large leaf net (not a hand skimmer!). This will stir up the water, and may make your pool look much worse temporarily, but the stirred up debris will settle within a few hours. DO NOT ATTEMPT TO VACUUM THE POOL IF YOU CANNOT SEE THE BOTTOM OF IT OR IF YOU HAVE A LOT OF DEBRIS ON THE BOTTOM. YOU MAY CLOG YOUR SKIMMER, FILTER OR UNDERGROUND PIPES !
   
2. Adjust the pH and Alkalinity levels of the water using pH Plus, pH Minus and Alkalinity Plus. These levels must be within the proper ranges or the water will never clear.
   
3. Shock the pool. This means super chlorinating the water to kill off any bacteria and algae. If your pool is very dirty, it may need MANY gallons of liquid chlorine (shock) over a period of days before the water clears. Start off by adding 3 or 4 gallons, and if you see no results overnight, add 3 or 4 more gallons the next day. Continue this process until you notice the water changing color to either cloudy white, light green or clear. YOU CANNOT OVER SHOCK A POOL ! The more you add, the quicker it will clear ! Note...shock is available in liquid or powdered form, but we recommend using liquid (it works faster). You may also want to add a high quality algaecide to the water at this point.
   
4. Run your filter 24 hours a day and backwash 3 or 4 times a day for quickest results. Green or cloudy water will quickly clog a filter, therefore you may have to backwash your filter many times a day until the pool clears. THIS IS NORMAL ! Remember, you cannot over-backwash a pool filter. The more you run your pool, and the more you backwash the filter, the faster the pool will clear up. If you have a D.E. filter, remember to add new D.E. to the skimmer closest to your filter after each backwashing. A D.E. filter is more work than a sand filter, but the D.E. filter will clear a green pool about 50% faster than a sand filter.
   
5. If you follow these instructions and your pool does not clear up within 4 or 5 days, your filter may not be functioning properly. If this is the case with your pool, have the filter system checked out by a professional. The pool will never clear up if the filter is not working properly !
   
6. Once the water does clear up, you will most probably see debris on the pool floor (dead leaves, algae, etc.). If there is only a small amount of debris present, you may want to vacuum it up using your own pool vacuum system. If there is a lot of debris on the pool floor, you should have the pool professionally POWERVACUUMED. This will save a lot of wear and tear to your filter system, as well as assuring that no debris gets clogged in your pool pipe

Article from: http://swimming.about.com