A filtering device for an aquarium which can easily perform maintenance, inspection and repair of a pump is provided. The present invention is directed to a filtering device for an aquarium to be installed outside an aquarium 1. The device includes a casing 20 open to the atmosphere, a filtration zone Z2 formed in the casing for filtrating water W introduced from the aquarium 1, a discharge pump 35 for sending the water passed through the filtration zone Z2 to an outside of the casing and discharging it into the aquarium, a functional component zone Z1 formed in the casing and separated from the filtration zone Z2, and a functional component unit 3 having the discharge pump 25 and a unit frame 31 for supporting the discharge pump 35. The functional component unit 3 is insertably and detachably accommodated in the functional component zone Z1 of the casing 20.