There is some water worth fixing and some that is too far gone. You can “fix” green pool water by using appropriate chemicals and filtration if it has not been green and cloudy very long – as in weeks (not months on end).
Your pool may be a better candidate for a drain and acid wash if:
1.) It is a dense and dark green where you cannot see more than a foot or two into it.
2.) It has algae built up on the walls and surfaces in a “carpeting” affect.
3.) It has more than a few contractor size trash bags of debris at the bottom.
4.) You have a sand filter.
If you have #1 or #2 going on then I would suggest a drain and acid wash right off. If you have #3 and / or #4 can mean it takes a lot more time and money to turn it around and that money and time might be better spent on a drain and acid wash if you can get that done for just a little more.
To fix cloudy green water:
1.) Make sure your filter is running properly and the pressure is not too high. Backwash it if you have that option before starting this process.
2.) Make sure the skimmer baskets and pump basket are empty.
3.) Set your pool to suck from the main drain. Try to get approximately ½ the suction from the drain. You may need to shut off the skimmers ½ – ¾’s of the way each to pull more from the drain. This is important to get the cloudy water pulled out of the pool and through the filter.
4.) Shock the pool and add some algaecide or yellow out depending on the color of algae.
5.) Run the filter and check back once or twice per day to ensure it continues to operate effectively.
a.) If you have a DE filter you may need to backwash it once a day or even more often when you see the pressure get 5 or 10 psi higher than the level it is at when clean.
b.) If you have a cartridge filter you may have to clean it out if the pressure gets above 35 or gets more than 10 psi higher than it is clean.
c.) If you have a sand filter be prepared for this process to take a long time. You can try adding a couple of scoops of DE to help it filter better. A sand filter has about ½ the filtration of a cartridge filter and about 1/10th the filtration of a DE filter. Check it once a day and backwash it when the pressure gets 5 or 10 psi higher than clean.
6.) Repeat step 4 and shock the pool again every day or two as needed if the water is still green. The green should normally go away just turn cloudy within a day if you have the proper shock dosage. After that you are really using your circulation and filtration systems to clear up the cloudy water due to the shock and all the dead algae.
NOTE: If your filter pressure will not stay stable for a few hours of filtering you likely will need to do a full disassemble and cleaning on your filter elements. This does not apply to sand filters.