A lot of people here seem to be mentioning oversizing filters. That might be why my tanks are always crystal clear, even when I know they should be polluted all to hell. I use a Cascade filter on my 29 gal. that's meant for a 100 gal. tank, and I love it.
I also found that when I had my FW tank and I switched from standard light to a "plant" light, my water looked even clearer because of the shade of light I guess.