To provide an aquarium apparatus which adopts a circulation system by a simple overflow method, can avoid the generation of noises, and can ensure a circulation flow quantity necessary for purifying water.

A circulation pump 6 is operated to introduce water in an aquarium 3 from an overflow pipe 7 and a bypass pipe 8 into a filtration tank 4, and then supply the treated water into the aquarium through treated water-supplying pipes 5a, 5b. The upstream end 81 of the bypass pipe is submerged to suck and discharge the water by a siphon action in a fully flowing state. Most of a circulated water flow quantity is discharged from the bypass pipe by the water flow quantity adjustment of a flow quantity adjusting valve 83, and only an oil film, and the like, on the water surface are discharged from the overflow pipe 7.