Most fish kills that happen during the summer occur because there is not enough dissolved oxygen in the water. Warm water holds less dissolved oxygen than cool water, which is why these events tend to happen during the summer months. When there is not enough dissolved oxygen in the water, fish cannot breathe and they die, sometimes in large numbers.
The likelihood of fish kills increases in ponds and lakes with a lot of algae and bacteria. Algae growth is encouraged when excess nutrients from fertilizers, manure or sewage enter the water. During the day, algae produce oxygen through photosynthesis, but at night algae use oxygen. The naturally occurring bacteria in ponds and lakes that degrade organic matter also use oxygen. So on hot summer nights during algal blooms, there isn’t enough oxygen in the water to meet demand and fish kills occur.
Reducing the amount of fertilizer entering the water and installing an aerator can help prevent fish kills.