Wet / Dry Filters are a very common choice for the saltwater aquarist. They provide biological filtration and use an aerobic (living only in the presence of oxygen) filtration method, the more oxygen it gets, the better it works. A sump is nothing more than an extra water containment area usually set up underneath an aquarium where you can place equipment, provide the aquarium with various types of water filtration, add top-off water to the tank and perform other maintenance tasks. Sumps supply the skimmers with a consistent volume of water, incorporate a prefilter, have the needed barrier, in case the electricity goes out and the tank drains down to the lowest point of the overflow(s).