With a leveling control system or a automatic refilling system makes it easy and convenient to keep the water level in the aquarium constant. A small control computer with an optical or mechanical sensor constantly monitors the setted water level in the tank and, if not reached, activates a pump which pumps water from a tank into the aquarium. The devices are designed so that min. and max. Values are set. Likewise, visual and audible alarms can be added. An automatic refilling makes the care of an aquarium considerably easier, as evaporated water is always refilled and the water level in the aquarium always remains the same.