In this design, aquarium water is dripped or sprayed over a bed of substrate medium on which the nitrifying bacteria grow. The water is then collected and pumped back into the tank. The volume of substrate medium is generally as great as or greater than that in a comparable canister filter, and the trickle filter provides the added advantage of highly efficient aeration of the return water. Water is aerated by contact with air at its surface. As the aquarium water trickles down over the nitrifying substrate, it forms a thin film around the surfaces of filter medium and presents a large surface area to the surrounding air, allowing rapid and effective gas exchange.
Oxygen dissolves into the water from the air, and carbon dioxide escapes in the opposite direction.
The aerobic nitrifying bacteria receive the oxygen necessary for their activities, and the water that returns to the tank is saturated with oxygen and has released surplus carbon dioxide. Outside power filters, which typically hang on the back of the tank, although capable of providing both mechanical and biological filtration, are ultimately better as mechanical filters than biological filters. This is because the gravel bed of an undergravel filter simply provides a larger surface area for bacteria than the medium in a power filter. For lightly stocked tanks, an outside power filter may be adequate, but for tanks with higher densities of fish, more biological filtration is often needed. For this reason, outside power filters are often paired with undergravel filters.