Receptacle for maintaining aquatic organisms, either freshwater or marine, or a facility in which a collection of aquatic organisms is displayed or studied. The first display aquarium opened in Regent's Park, England, in 1853. Many of the world's principal cities now have public aquariums as well as commercial ones; other aquarium facilities serve chiefly as research institutions. Regardless of sizewhether a small one-gallon jar or a huge million-gallon tankaquariums must be constructed with care; many substances, especially plastics and adhesives, that are nontoxic to humans are toxic to water-breathing animals. The primary requirement for maintaining aquatic organisms is water quality.