The aquarium creates a smooth circulation of water flowing through mechanical, biological, and chemical filtration mechanisms. The three filtration mechanisms are housed in a vertical first compartment and a vertical second compartment, both along a back side of the aquarium body, and a lateral third compartment along a bottom side of the body, respectively. A water pump draws water from the third compartment into the second compartment. After a first filtration processes, the water is poured back into the body. Due to the suction of the water pump, water flows from the body into the second compartment to undergo a second filtration process and then enters the third compartment, where a third filtration process is conducted. The water is finally drawn into the second compartment to complete the circulation.