Nitrate, the end product of the nitrification process, tends to accumulate in aquaria systems. Removal of nitrate in these systems by means of biological denitrification requires the addition of an external electron donor. In this study, the possibilities for using hydrogen gas in aquaria systems as a harmless alternative to other electron donors such as sulphur or reduced organic carbon was investigated. To circumvent safety issues regarding the storage of large volumes of hydrogen gas, in situ generation of hydrogen gas by means of a separate electrochemical cell was chosen. A plug flow reactor filled with polyester cotton wool as carrier material for denitrifying bacteria received hydrogen gas in the headspace. Removal rates of 15 +/- 6 mg N per day were achieved with a hydraulic residence time of 3.3 hours. During the start-up phase nitrite peaks up to 3 mg N per liter were measured in the effluent of the reactor.