Originally developed for public aquariums exhibiting tropical reef-dwelling organisms, smaller versions of these effective filters are now marketed for use by hobbyists. The mechanical part of the filtration process is performed by a pad or other material through which the water first passes. The main role of the wet/dry filter, however, is in biological filtration. The area upon which the beneficial bacteria can grow is increased through the use of materials with a large surface area. Most recently, ceramic tubes or partially hollow plastic balls, called bio balls have been used for this purpose. These materials are not submerged, but rather a trickle or spray of water from the aquarium is directed over them. Nitrifying bacteria grow so well in a well-managed wet/dry system that their nitrate production can actually be detrimental to the aquarium's inhabitants. This danger can be lessened by the installation of another chamber that supports anaerobic bacteria. These bacteria convert nitrates, the end product of the nitrogen cycle, to nitrous oxide and free nitrogen. The free nitrogen is then lost to the atmosphere.