How does eutrophication cause fish to die

One of the negative impacts of eutrophication and increased algal growth is a Loss of oxygen known as anoxia

Significant fluctuations in dissolved oxygen concentrations between day and night can occur in waters where there is enhanced plant growth. This can cause problems in the early morning with low concentrations of oxygen levels. The result of plant respiration and the death of invertebrates and fish. This process can be compounded. when algae blooms through their decay further reduce the oxygen content