As the phytoplankton die and decompose, the remaining oxygen in the pond is consumed and within a short time, a fish kill occurs due to oxygen depletion. To prevent oxygen shortages due to phytoplankton die-off, proper fertilization should be practiced. A herbicide can be used to thin out phytoplankton growth when it becomes too dense. Caution should be used to prevent a fish kill. If oxygen depletion occurs, apply either triple superphosphate fertilizer at the rate of 17 pounds per acre, or liquid fish pond fertilizer at one gallon per acre to re-establish the phytoplankton.
Overturns can lead to oxygen depletion fish kills.
The colder water may become oxygen-deficient due to interaction with certain elements in the pond bottom and lack of photosynthesis by phytoplankton at greater depths. An overturning or mixing of this colder, oxygen-deficient water may occur after a heavy rainfall. If the rainfall is of sufficient quantity, it produces a mixing of the pond water as the cold rainwater sinks to the pond bottom. It displaces the oxygen-poor water, which mixes with the water in the upper levels of the pond. The result is a fish kill due to insufficient oxygen in the water.