Context In Australia’s Murray–Darling Basin, the invasive common carp (Cyprinus carpio) poses a significant ecological threat. Efforts to manage their population include the potential use of cyprinid herpesvirus (CyHV-3) as a biological control agent. The environmental benefits of carp elimination could be substantial in the long term; our knowledge of the immediate ecological consequences resulting from large-scale fish die-offs remains limited. Aim This study sought to investigate the effect of a large-scale fish-mortality event on water quality. Methods Experiments of an increasing scale (bucket, mesocosm and whole wetland) were conducted to assess how decomposing carp carcasses alter dissolved oxygen and nutrient concentrations in water. Key results In mesocosms, dead carp decayed more rapidly at 18 than 12°C, yielding oxygen demands of 1.022 ± 0.029 and 0.496 ± 0.239 mg kg−1 min−1 respectively. This carp decay released phosphorus, yielding 2121.1 ± 140.4 mg kg−1. In the wetland experiment, carp addition of 2400 kg ha−1 resulted in anoxic conditions over the following 2 weeks. The release of dissolved organic carbon and lipids led to a peak biological oxygen demand of 95.3 mg L−1. Conclusions Carp decomposition considerably contributes to biological oxygen demand and algal growth through nutrient enrichment and is strongly influenced by carp density. Implications The finding highlights key factors to consider before using biocontrol agents causing mass carp mortality, including hypoxia, anoxia and increased risk of harmful algal blooms.