There are various setups that can be incorporated with a wet/dry filter system but the basic components consist of a pre-filter, bio-filter, and sump. An aquarium tank is setup so that water overflows through a tube which leads to the sump. The sump is an external tank which contains the pre-filter and bio-filter. Water flows to the top of the sump on one side where it goes through a pre-filter element such as a sponge or cloth to trap bigger debris. The water then flows or trickles down over a bio-filter media such as bio-balls. These plastic bio-filter media are specially designed so that natural bacteria can be produced. The trickling water over these bio-filters creates large amounts of air which helps the bacteria grow. Such bacteria is commonly referred to as beneficial bacteria as they consume the toxins that are produced by the fish.
Once the water goes through the bio-filter, it collects at the bottom of the sump where a water pump redistributes the water back into the tank. As the basic concept behind the filtration is all natural, the process is highly efficient and requires less maintenance.
Wet/dry filters rely on the growth of bacteria, therefore, use of this system in a new tank requires care. This includes letting the water filter through the tank for several weeks in the presence of a food source for the bacteria.
A small number of hardy fish can be introduced to the tank to act as a source of ammonia while the bacterial colony grows large enough. During the maturation process the fish should be closely observed and, if possible, the water tested for ammonia and nitrite and corrective action taken when toxic levels are reached.
Bottled ammonia can be used as an alternative to hardy fish and such "fishless cycling" of a new aquarium is increasingly being recommended by responsible fish keepers.