A system for effective removal of surplus food and fecal residues in fish rearing tanks is described. The system utilizes the siphon principle known from laboratory pipette washers, and is designed to work with the outflow of water through the center bottom outlet of the tank. When the siphon system is at work, all accumulated debris will be removed into the drain. A total emptying of tank by the siphon is avoided by breaking the outflow water column by an inlet of air. Air is introduced into the water column of the siphon at an appropriate water level by a narrow tube connected through the sidewall of the tank. Combined with a circular water current in the tank and a declining bottom toward the center, the system will effectively remove organic waste accumulated within each filling cycle of water. The biological advantages from periodic changes in water speed within fish rearing tanks is discussed.