The effects of the nutritional composition of the substrate on the bacterial growth and their N mineralization after adding organic carbon (glucose) and organic nitrogen (glycine) to the experimental fish pond were investigated in this paper. The study showed that the growth rate of bacterial assemblage and their uptake of N were enhanced by enriching the water with organic carbon. On the other hand, the growth rate of bacterial assemblage was not significantly increased, while the N mineralization was increased by enriching the water with organic nitrogen. The nutritional status of fish pond water implied that the bacterial growth was limited by the content of DOC in the substrate. The bacterial assemblage in water of the experimental fish pond acted as a decomposer of organic nitrogen in the process of nitrogen cycling within the fish pond ecosystem.