Hi,
Remove any carbon or such like from the fluval, and replace with bio chem zorb, or another product that does not leak whatever it has removed from the water, back into the water, once it is "full", if that made sense.
Sounds more like an algae bloom. I get the same simptoms sometimes after feeding Zucchini.
I can feed massive amounts of food and no problems. One bit of Zucchuini and "cloudy city" the next day.
How often are water changes being done? and how heavy is the fish stocking?
Remember that the zucchini (once it hits the water) is really just decaying plant material, so I would also suspect that there may be some decomposing plants in the tank somewhere that are not helping.
As to the fluval slowing down, I have experienced something similar on occasion. What I found was the round part (with the holes in it that leads back to the out hose) in the cover that sits on top of the media trays was actually getting blocked by material in the top media tray. Turning the filter off to investigate, would show nothing because as soon as the filter was turned off, the material that was causing the blockage would settle back into the tray.