An occasional problem that confronts pond owners is the loss of part or all of the fish population in their ponds because of a fish kill. Fish kills can eliminate or severely reduce the sport fishing provided by ponds. A basic knowledge of the causes and symptoms of fish kills and possible remedial measures will be advantageous to the pond owner. One of the most common types of fish kills in ponds is caused by depletion of dissolved oxygen in the water. Oxygen must be present in the water at or above certain minimum concentrations for fish to survive.
Phytoplankton imparts a green color to the water in a properly fertilized pond.
It is essential as part of the food chain and as a supplier of oxygen. A sudden die-off of these plants may occur, especially during periods of cloudy, overcast weather, when the plants use more oxygen than they supply to the pond. 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.