Outdoor mesocosm experiments in a reservoir were used to examine the changes in water quality and the response of bacteria and algae to increase loading of dead fish biomass that simulated a release of the Cyprinid herpesvirus-3 (CyHV-3) as a biological control for carp. A series of replicated manipulations conducted between November-December 2016 demonstrated the effects of a large fish kill at different biomass concentrations. We found dissolved oxygen dropped to zero within four days in all treatments. There was an increase in E.coli and total coliforms in all treatments with dead carp. There was an increase in total nitrogen and total phosphorus with increase in biomass of dead carp. There was also increase in total algal counts in all treatments with dead carp. The excess in nutrients from the dead fish was utilised by algae and dominated by Chlamydomonas sp. followed by Scenedesmus sp. The timing between bacteria dominated and the algal dominated was determined by the amount of fish biomass. These results show that a fish kill at high biomass volumes can have serious detrimental effects on water quality over the first month.