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