I had the same problem so I went through all the steps. Water change, filter clean, and cleaning of the sand. The water would be clear for a little bit then as time went on it just got greenner and greener. So after all that it turns out to just be a algae boom cause I had my water tested and the lfs SAID " your water is perfect' .
I fixed the problem with a box of algae tablets by 'Tank buddy" !