Aquarium, a tank or container that is filled with water and stocked with aquatic organisms, especially fish; also an establishment in which these organisms are exhibited. For the home, freshwater aquariums are preferred to saltwater aquariums. Saltwater aquariums are more difficult for the beginner to maintain because the saltiness and acidity of the water must be frequently tested, and because saltwater fish are more sensitive to chemical and temperature changes than freshwater fish.

Home Aquariums

Home aquariums range from simple goldfish bowls to elaborate saltwater aquariums with sophisticated filtering and monitoring equipment. The average hobbyist can find adequate equipment and instructions on maintaining a small freshwater aquarium at tropical fish stores.

The first step in preparing an aquarium is to clean the tank with a strong salt solution. Soap or detergents should not be used because they might leave a harmful scum. The bottom is covered with gravel or coarse sand that has been thoroughly washed. Although it is possible to set up a balanced aquarium with few, if any, mechanical devices, most home aquariums are equipped with such apparatus as a filter, air pump, aerator, and heater. Artificial or real plants, as well as rocks, are usually added to provide shelter and a natural setting for the fish.

Scavengers are helpful, but not necessary, for keeping the aquarium clean. Small catfish, called Corydoras, are the most satisfactory scavengers; the large Japanese mystery snail is also an effective cleaner.

Goldfish and small tropical fish are the most popular kinds kept in home aquariums. They should not be kept together, however, as their temperature requirements differ. The aquarium should not be crowded. The water should be clean, free from chlorine and other chemicals, and at a constant temperature.

Public Aquariums

There are many public aquariums throughout the world. Most of them are arranged so that visitors can view the aquatic life through windows. The windows usually flank a hallway, and in general there is a single tank behind each window. In this way, the various types of fish and other aquatic animals are separated. Many aquariums recreate diverse aquatic environments, such as coral reefs, river bottoms, or various coastlines, in large tanks. Among notable American aquariums are the John G. Shedd Aquarium, Chicago; the National Aquarium, Baltimore; the New York Aquarium, New York City; the Steinhart Aquarium, San Francisco; and the Aquarium of the Americas, New Orleans. Many zoos also have aquariums.

Some aquariums have oceanariums—huge tanks that allow visitors to view marine animals from above as well as from the sides. Here there is no attempt to separate the various kinds of marine animals. Various marine environments are often recreated. Popular oceanariums include those at the Miami Seaquarium, in Miami, Florida; and the Monterey Bay Aquarium, in Monterey, California.

Public aquariums often carry on various types of scientific activities. These activities include maintaining breeding and survival centers for endangered species and gathering scientifically important data about the various plant and animal species in their collections.