Dirty Water

I have a question about filtration. I have a 4ft 50g tank, and no matter
what sort of filtration I apply, it always seems dirty. Whenever the
gravel at the bottom is disturbed, the water becomes filled with dirt
particles, which then go on to attach themselves to the leaves of the
plants to the detriment of them.
I am currently using a cannister filter which should easily be powerful
enough, and for several weeks even had a box filter, sitting on the side
of the aquarium, none of which made any difference.
I currently have six medium sized Cichlids.
Can someone suggest a form of filtration that would suit the tank, and
leave the water and the gravel clear of dirt and debris??
-- Alex
Alex Shaw : ashaw at ssg_com.au