Abstract: A filter device for an aquatic tank includes a filter unit having a first casing. The first casing has a top open end, a bottom closed end formed with openings, a surrounding side wall interconnecting the top open end and the bottom closed end and being formed with a water inlet opening, and a perforated first tubular member extending from one of the openings of the bottom closed end to the top open end. The filter unit has a first filter medium provided around the first tubular member. A bottom basin has a top open end. The bottom closed end of the first casing is stacked on and detachably interengages the top open end of the basin. A top pumping unit includes a second casing and a pump mounted in the second casing. A bottom end of the second casing is stacked on and detachably interengages the top open end of the first casing. The pump has an entrance port connected to the first tubular member and an exhaust port extending out of a surrounding side of the second casing.