A pond turnover is a natural event that can occur in any pond. The oxygen-rich upper layer of the pond drops and mixes with the bottom layers of the pond, which lack oxygen and are full of toxic gasses and muck. This reduces the overall oxygen levels that are often times insufficient for fish survival.
Some signs of a possible turnover include significant darkening of the water and fish gulping at the surface. Darkening of the water does not always mean that you are experiencing a turnover. Not every pond will experience a turnover, and the severity of a turnover can vary. There can be anywhere from a few fish to a few hundred effected. Often times with turnovers, the fish who are gravely affected are of similar size.
There is no exact way to prevent or stop a fish kill once it has started. Excessive rainfall, fluctuating temperatures, extreme heat, and the change of seasons are just a few factors that can cause a turnover. Since these are all situations that we can’t control, there is very little one can do. Leaving on your aerator or providing fresh water to the pond are ways to help provide more oxygen to the water, but is not a guaranteed fix.