Includes: Controller, pH-sensor, sensor holder, pH-test fluid and solenoid valve. The pH controller is a precision instrument for controlling the pH value in aquaria. In combination with a CO2 system and a solenoid valve the pH can be maintained at a constant level, and the CO2 supply to aquatic plants can be controlled. The unit is supplied with a large LED display and the control range is digitally adjustable from pH 3.5 to 10.5. The direction of adjustment can be set up or down.