When you vacuum your pool, you need to vacuum to waste. Do not vacuum to the sand filter. It will infest it with dead algae. After vacuuming to waste, backwash your filter just to make sure you get rid of any algae in your filter. Then go to rinse and run in the rinse cycle for at least 30 seconds. When water is clear, go back to filter. Green water should not be coming out of the return. Also, make sure you turn the pump off each time you change the handle between filter, waste, backwash, etc.
It will take a while, but your pool will clear up. Don't forget to brush your pool aggressively after vacuuming. You also need to maintain a high chlorine level based on your CYA numbers to insure you've killed off the algae.
With a sand filter, always vacuum to waste when dealing with lots of dead algae.
Hope this helps.
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