Overall water splitting to form hydrogen and oxygen over a heterogeneous photocatalyst using solar energy is a promising process for clean and renewable hydrogen production on a large-scale. The ZnGeN2–ZnO solid solution photocatalyst shows photocatalytic activity under visible irradiation. This paper deals with results of photocatalytic reactions over the (Zn1+xGe)(N2Ox) photocatalyst for overall water splitting. The photocatalyst shows high rates for overall water splitting under visible irradiation, with notable deactivation with time. This study focuses on measuring rates for H2 and O2 evolution to elucidate the cause of the deactivation. Our results showed negligible changes in the structure and the composition of the photocatalyst after the photocatalytic reaction, and thus the changes undetectable by the techniques employed should be the cause of deactivation. This paper also discusses two strategies to improve the photocatalytic activity: metal doping in the oxynitride formulation and post-calcination after nitridation. Several characterization carried out in this study led us to conclude, at the current point, that the improvement of photocatalytic activity is ascribed to the reduction in the number of defects in the photocatalyst materials.