This condition may develop into a major fish kill when sudden summer rains occur. These rains will cool the warmer upper layer of water enough to allow it to mix with the oxygen poor layer below. Decomposing materials in the oxygen-poor layer are again mixed evenly throughout the pond, resulting in an overall reduction in the dissolved oxygen level. Fish previously able to avoid the oxygen depleted layer are now susceptible to low-dissolved oxygen syndrome and possibly death. Heavy algae blooms coupled with extended periods of cloudy weather or thunderstorms are all that is needed to trigger oxygen depletions.
Cloudy weather can lead to algae die-offs while thunderstorms can cause pond turnover.
An early warning sign of impeding problems is a color change in the water from a rich green to a tea color. Another more acute signal will be fish swimming lethargically at the surface of water. By the time you see the fish at the surface it is usually too late for the farm pond owner. The fish are stressed to the point that some, if not most, of the fish will die. At best only the large fish will die.