I have a similar problem with my 450l 4' tank with particles in the water, just looks messy almost all the time. I have a 303,304,Shark 3 and a atman 104 which also acts as the pump for my UV filter. My PH was always low because of drift wood, so I used argonite(spelling) in one of my filters and just kept adding more over 2 weeks until it settled around 7. All water tests are nil. And remembering weekly gravel cleans helps heaps as my Oscars and pleco are messy.
With a tickle filter I have seen another piece of glass going from slightly higher then the trickle filter glass top to 3/4 of the way down to the bottom with a slight gap between glasses to take water from lower down in the water colomn, would also help I believe. This was on a crystal clear 8' tank (was however under stocked for size of tank)
Colin