According to the Virginia Department of Game and Inland Fisheries, pond fish may die for a number of reasons, including low levels of dissolved oxygen, pollutants, disease, temperature extremes or natural causes. The department explains that when fish in a pond begin to die or languish, it is imperative for pond managers to review these common factors to ensure that they are within recommended parameters. Often, the conditions causing the problem can be corrected.
According to the Warnell School of Forestry Resources, a division of the University of Georgia, Athens, oxygen depletion is a result of demand outstripping the pond’s supply. While the micro organisms and fish in the pond consume oxygen, the only organisms that produce oxygen are the plants. Additionally, the movement of air over the water can increase the amount of dissolved oxygen in the pond. Frequently, according to the Warnell School of Forestry Resources, fish kills occur during the summer, when pond managers overstock the pond or treat it with weed-killing chemicals -- both of which may reduce the available oxygen.
Outdoor Alabama cautions pond managers to be observant for the signs of low dissolved oxygen in ponds, such as fish swimming near the surface or gulping air. Usually, the largest fish die first.