Disclosed is an aquarium(1) which people can observe an ecosystem of a river. Aquarium(1) comprises a wall made of a glass(2), a circulating means including a water supplying and discharging devices(4 and 5), a means for regulating flow rate of the water such that the flow rate can be varied when the water is circulated from one portion of aquarium(1) to the other portion of aquarium(1), a means for forming an environment of aquarium(1) such that the environment of aquarium(1) is identical to that of the river and a means for generating a swirl-shaped flow in aquarium(1) which increases an oxygen content of the water.
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