A cleaner for an aquarium including a filter, a water pump, a waste-pipe with one end connecting to the water pump, an air compressor, a hose with one end connecting to the air compressor, and a base assembly. The base assembly includes a rectangular box having a top sieving plate, a V-shaped gatherer disposed in the box, a cylinder clamped in the box between the sieving plate and the gatherer, a rotatable cleaning brush rotatably disposed in the cylinder, and an actuating paddle wheel fixed to one end of the rotatable cleaning brush. A number of sieving slits are provided on the sieving plate. A hole is provided on the cylinder for receiving the other end of the waste-pipe, and another hole is also provided on the cylinder at its lower surface near one of its ends for receiving the other end of the hose, and a number of notches are formed on the lower surface of the cylinder for passing the depositing impurities. The rotatable cleaning brush pushes the impurities in the cylinder to one end of the cylinder close to the room under the first hole of the cylinder. The actuating paddle wheel is driven by the air bubbling up from the hose so as to drive the rotatable cleaning brush.