To provide a water tank-maintaining device simplified in the structure, capable of surely lighting a cold cathode-ray tube, easy in illumination, sterilization and water purification, and useful for breeding aquarium fishes, etc., by disposing a capacitor in series between the cold cathode-ray tube side end of a connection cord and one of the electrodes.

This device for maintaining a water tank is provided with an electric source 1 for outputting an AC electric power, a connection cord 3 for supplying the AC electric power, and a cold cathode-ray tube 5, such as an UV lamp, lightened with the AC electric power supplied through the connection cord 3. A capacitor C2 is disposed in series between the cathode-ray tube side end of the connection cord 3 and one of the electrodes of the cold cathode-ray tube 5. UV rays emitted from the cold electrode-ray tube 5 are preferably utilized for the generation of ozone to purify water in a water tank.