Berlin

The Berlin Method of biological filtration is method for keeping an aquarium, typically a saltwater aquarium, clean.

It is basically the use of ample live rock (rock with live marine organisms and bacteria on or in it) in a refugium or tank.

Most often a separate tank containing a large amount of live rock but without fish or corals is linked to the main aquarium via pumps and tubes. No substrate is used in a Berlin setup to facilitate the vacuuming of debris, although a deep sand bed, biological filter setup may be used in conjunction with the Berlin. A protein skimmer is used to remove organic solids. The theory is that anaerobic bacteria deep within the rock convert nitrate to free nitrogen gas.