Abstract: An aquarium includes a lower hollow body which defines a lower chamber and an upper hollow body which defines an upper chamber. A mount plate is removably mounted in a bottom of the lower chamber and a waterproof housing is provided on the mount plate to receive a pump therein. The upper hollow body includes a plate and a housing projecting upward from the plate to define the upper chamber. A water inlet is formed in the plate beyond the housing, through which water is inputted into the lower chamber. An air passage is formed in an upper wall of the housing. A float is movably provided to the housing to block the air passage for isolating the upper chamber unless the float is raised. A protrusion projects upward from the upper wall of the housing, defining a cavity above the upper chamber. An inlet pipe has a lower end attached to the pump and an upper end extending into the cavity defined by the protrusion, preventing water from entering into the inlet pipe.