The present invention relates to a filtering apparatus including a filter device externally hitched on a wall surface of an aquarium. A water pump pumps water stored in the aquarium into the filter device. The filter device is formed so that the water pumped from the water pump is divided into a first flow which is returned into the aquarium unfiltered, and a second flow which is filtered through a filter element and then returned into the aquarium in dependence on a siphon effect.