I don't have a tank yet, but have been facinated with saltwater aquariums that I have seen and am thinking about taking the plunge, so to speak. I've been looking at equipment and different systems of filtration, knowing that the nitrogen cycle is important. Tonight I stopped at Aquarium Concepts in no. Seattle and they had a 5-8" deep sand type filter in lieu of wet-dry plastic balls for bio-filtration. Whats the difference?