I have a 6 x 3m pond with approx 30- 40 fish including large koi. Since
about August last year I am unable to get the water clean in the pond.
You can not see fish if more than 2 inches below the water level.
I use nautilus 8000 pump,
Green machine filter system
vortex 18000 uv.

I have tried allowing the pump to pass larger solids by removing the cage.
Inserted foam sheet into the filter box which turn black with hours.
Slowed down the rate the water passes
And yet i can still not get the water clean.

Can anyone suggest anything else I can try.