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Generic schematic of a basic aquatic life support system. Mechanical filtration should precede biofiltration. Mechanical filters help prevent channeling and anoxia in biofilters by removing large particulate matter. Biofilters must have a lot of surface area for nitrifying bacteria. Chemical filtration, although not required in all designs, removes any color compounds and cleans water before it is returned to the vessel housing the fish.
Courtesy of Dr. Ruth Francis-Floyd.