The most common cause of fish kills in Ohio is suffocation due to lack of oxygen. A turnover results when a strong wind, rapid temperature change, or inflow of a large volume of cold water causes the upper layer of water to be replaced by the lower layer of oxygen -deficient water. The bottom, cooler layer of water in the pond has the potential to hold more oxygen than the top layer because it is denser. However, the lack of photosynthesis and the decomposition of organic matter actually results in a lower level of dissolved oxygen compared to the top layer. Using an aerator, controlling weeds so they don't decompose, and keeping snow off the ice in the winter are all ways to prevent oxygen depletion.
Plants cease photosynthesizing at night but continue to utilize oxygen for respiration.
Thus, oxygen levels are lowest just before sunrise and highest at midday. In some cases, cloudy, cool, or rainy weather can cause algae scums to die. The death and rapid decomposition of these algae can reduce dissolved oxygen content from 8 to 0 parts per million within 24 hours. Once a fish kill is in progress, little can be done to stop it. Aeration should be provided, but sometimes even aeration cannot overcome the oxygen deficit.