This is a small round aquarium with two fishes. An aquarium is used to keep fish, invertebrates, amphibians, aquatic reptiles, such as turtles, and aquatic plants. The aquarium principle was fully developed in 1850, when it was concluded that plants added to water in a container would give off enough oxygen to support animals, so long as the numbers of animals did not grow too large. An aquarist owns fish or maintains an aquarium, typically constructed of glass or high-strength acrylic. Cuboid aquaria are also known as fish tanks or simply tanks, while bowl-shaped aquaria are also known as fish bowls. Size can range from a small glass bowl, a few liters in volume, to immense public aquaria of thousands of liters. Specialized equipment maintains appropriate water quality and other characteristics suitable for the aquarium's residents.