A type of filtration, used in aquariums, that makes use of beneficial bacteria that colonize any available underwater surface. These bacteria drive the nitrogen cycle by converting highly toxic ammonia into less toxic nitrites, and then converting nitrites into even less toxic nitrates. The nitrates are removed by plants or regular partial water changes. An efficient biological filter will have something with a large surface area for the bacteria to colonize, like a sponge filter or under gravel filter. Legos work for biological filter media as well.