The maintenance of an aquarium of any size requires the careful regulation of water flow, temperature, light, food, and oxygen, removal of injurious debris, and attention to the special requirements of the individual species kept. Green aquatic plants are often used in aquariums since, through the process of photosynthesis, they utilize waste carbon dioxide from the animals' respiration and in turn provide oxygen. An aquarium in which the dissolved gases are kept at the proper concentrations by the physiological activities of the plants and animals is called a balanced aquarium. Certain mollusks, such as snails and mussels, are useful as scavengers, as are some species of fish.

Large freshwater and saltwater aquariums are often maintained for research and breeding purposes by universities, marine stations, and wildlife commissions, e.g., those in Naples, Italy; Monaco; Plymouth, England; La Jolla, Calif.; and Woods Hole, Mass. There are also many aquariums throughout the world for public exhibition. Among those in the United States are the Aquarium for Wildlife Conservation (formerly the New York Aquarium) at Brooklyn, N.Y.; the Georgia Aquarium at Atlanta; the John G. Shedd Aquarium at Chicago; Marineland of Florida at Marineland, Fla.; the Monterey Bay Aquarium at Monterey, Calif.; the National Aquarium at Baltimore; the New England Aquarium at Boston, Mass.; the New Jersey State Aquarium at Camden; the Scripps Institute of Oceanography at La Jolla, Calif.; the South Carolina Aquarium at Charleston; the Steinhart Aquarium at San Francisco; the Tennessee Aquarium at Nashville; and the Waikiki Aquarium at Honolulu.