An energy-saving device of water-level regulator without using the electric pump was developed based on the siphon action to drain water from the bottom of aquarium. The siphon action was triggered by draining water from a water-level regulator at the beginning. The water from an aquarium could consequently flow into the box of regulator, so that water-level of the aquarium could be maintained constantly. In addition, experiments on drain efficiency were conducted, and the water of aquarium was filled with a flow rate of 25, 50, 75 and 100 ml/sec, respectively. The results showed that the water-level of each trail was constant at 0.4, 1.5, 3.4 and 5.1 cm above datum point. For the purpose of saving energy and efficiency concerns, this device can be used in a series of aquaria with a water-recirculating system without using many set of filters and electric pumps.