A custom made trickle filter used to filter a multi-tank system.
Trickle filters only have about 4 inches (10 cm) of water in the bottom of the filter sump allowing the biological media to be exposed to air. The advantage of removing the biological surface area out of the aquarium and exposing it to air is enormous, as it provides beneficial bacteria with a highly oxygenated environment. Bacteria in trickle filters can reach high population densities on the media. Beneficial bacteria thrive in this environment and break down ammonia and nitrite much quicker than the bacteria in the aquarium.
Because trickle filters have excellent biological filtration capability, they are often used on systems with high density stocking levels, like rearing systems. Trickle filters are also often used as a central filtration system for multiple aquariums.
Adding an airline to the bio-media chamber to pump in fresh air from an air pump can help get fresh oxygen to the bacteria. Most trickle filter builders neglect to add this feature to their filters, but you can modify the filter quite easily by drilling a hole in the side (if made of acrylic) of the bio-media chamber, and cement a hard plastic air tube going directly into the middle of the bio-media. Without this air being pumped into the bio-media chamber, gas exchange can limit the potential efficiency of the filter.
Sump filters are the evolution of the trickle filter. Sump filters typically use a micron bag to filter the particulate from the aquarium water. Sump filters do not use bio-media, as the intent in this design is to let biological filtration to take place on the surface area within the aquarium. Water is oxygenated while it passes through the bag filter. Like the trickle filter, sump filters are powered by either an internal sump pump or external pump.