What Should I Use? - A Berlin Sump , a Wet-Dry Filter, or a Refugium ???
Definition of Sump
In the world of aquariums, "Sump" or "Berlin Sump" is a general term used for an open basin or container of water placed below the aquarium which is fed water from the surface of the fish or reef tank. Water is then delivered back to the aquarium above, using a pump connected to or inside of the sump. A sump can also be the part of a filter system that also has an open area for pumps and/or protein skimmers and other devices used to improve or monitor water conditions.

Sumps for aquariums
Aquarium Sumps can be classified into three categories, Berlin Reservoirs, Wet-Dry Filters and a specialized type of system called a Refugium. These systems may vary widely in design, shape and mechanics, a fact that prevents us from being able to provide generic set-up instructions. However, almost all are typically a large box-like acrylic or glass tank that has been divided in the center into two sections. One section is where water from the aquarium is delivered. The other section is used to allow a pump to return water back to the aquarium.

Berlin Sump System
The word "sump" usually refers to the section that contains the pump or pump intake. However, in the case of a Berlin system, which is a completely open tank, the entire reservoir is sometimes referred to as the "sump."All of these systems have several things in common. Because these filters are open systems, they can only be fed by water drawn from the surface of the aquarium. If they were fed from below the surface, the filter below would overflow and flood when the pump was turned off. Therfore, they all require the use of an overflow prefilter of some sort, either built-into the tank or of a design that hangs on the back of the aquarium.

Which Do I Use?
The big question we always are asked is, which to use? In general, if the aquarium has live rock in the aquarium or in the sump, then a Wet-Dry Filter is not nescessary, as a matter of fact it is just a holding area for detritus (garbage from the tank), and not really desired. The biological surface area on the live rock is awesome for the good bacteria to form and adhere to, and provides the best possible biological system growth. Tanks with live rock for biological media usually are unscathed by large fish loads, since the biological system is so strong.
If the aquarium being built is a fish only system live rock is not desired, then a wet-dry filtration or sump system is very nescessary to grow the needed biolobical media for a stable aquarium environment.

How about a refugium?
A refugium is a sump where a partitioned area has been added to grow marine plants that absorb the nitrates in the water. These nitrates act as fertalizer for the plants, and keep the nitrate level in your aquarium at 0 or very close to it. A refugium can be a second sump system added to the aquarium, or it can be the main sump system, with a partitioned area dedicated for the main pump, a dedicated lit area for the marine algae, and a dedicated area for a protein skimmer.