and tanks housing cichlids have to take species aggressiveness into account when stocking. specimen tanks usually only house one fish species, along with plants—sometimes those found in the fish species' natural environment—and decorations simulating a natural ecosystem. this type is useful for fish that cannot coexist with other fish, such as the electric eel, as an extreme example. some tanks of this sort are used simply to house adults for breeding. biotope aquaria is another type based on species selection. in it, an aquarist attempts to simulate a specific natural ecosystem, assembling fish, invertebrate species, plants, decorations and water conditions all found in that ecosystem. public aquaria often use this approach. biotope aquaria simulates the experience of observing in the wild. it typically serves as the healthiest possible artificial environment for the tank's occupants. public aquaria the 80-meter (260 ft) underwater tunnel in aquarium barcelona lisbon oceanarium designed by architect peter chermayeff the baltic sea aquarium at the maretarium in kotka, finland most public aquarium facilities feature a number of smaller aquaria, as well those too large for home aquarists. the largest tanks hold millions of gallons of water and can house large species, including sharks or beluga whales, which typically couldn't be housed properly in the home aquarium. dolphinaria are specifically for dolphins. aquatic and semiaquatic animals, including otters and penguins, may also be kept by public aquaria. public aquaria may also be included in larger establishments such as a marine mammal park or a marine park. these are very popular around the world, especially with a new emergence in the middle east. virtual aquariums a virtual aquarium is a computer program which uses 3d graphics to reproduce an aquarium on a personal computer. the swimming fish are rendered in real time, while the background of the tank is usually static. objects on the floor of the tank may be mapped in simple planes so that the fish may appear to swim both in front and behind them, but a relatively simple 3d map of the general shape of such objects may be used to allow the light and ripples on the surface of the water to cast realistic shadows. bubbles and water noises are common for virtual aquariums, which are often used as screensavers. the number of each type of fish can usually be selected, often including other animals like starfish, jellyfish, seahorses, and even sea turtles. most companies that produce virtual aquarium software also offer other types of fish for sale via internet download. other objects found in an aquarium can also be added and rearranged on some software, like treasure chests and giant clams that open and close with air bubbles, or a bobbing diver. there are also usually features that allow the user to tap on the glass or put food in the top, both of which the fish will react to. some also have the ability to allow the user to edit fish and other objects to create new varieties.