Filter, 24/7. Put a little DE (about 1/4-1/3 cup) in the skimmer after you backwash. Add some more in similar small amounts to get the pressure to go up 1-2 lbs. Repeat after backwashing.
Get some "Skimmer Socks" or similar and put one in the skimmer. They pull out an incredible amount of stuff.

If that still doesn't start to improve it, try adding about 1 cup of polyquat 60% "algaecide"--only polyquat. It will drive your FC down but just add chlorine to raise it back up. Polyquat is an algaecide that acts as a clarifier.

Be patient.