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Troubleshooting Fish Kill Problems


Cause of fish kill



Recommended Solution(s)

Summer kills

Cloudy, hot, still days and nights. Low oxygen levels in pond water. Fish found dead and/or gasping for air at the surface;

Water temperature reaches very high levels (>85° F) in shallow ponds; very warm water does not hold as much oxygen. Cloudy skies prevent plants from producing oxygen and calm winds keep oxygen from mixing into surface water. Shallow, weedy ponds are especially vulnerable.

Deepen pond and/or install aeration system.


Low oxygen levels in pond water. Dead or gasping fish found after a violent thunderstorm which produces heavy downpours and high winds.

Large sudden inflows of cool rainwater and strong winds cause bottom water (low in oxygen) to up-well and mix with the surface water resulting in critically low oxygen levels. More likely to occur in shallow, weedy ponds with large, steep drainage areas which produce high runoff.

Deepen pond and/or install aeration system to circulate and aerate bottom water that lacks oxygen.


Phytoplankton die-off

Low oxygen levels in pond water. Fish found dead and/or gasping for air. Pond water with a green cast prior to or during fish kill. Phytoplankton may look like green paint floating on the water surface.

Nutrient enriched ponds produce dense blooms of phytoplankton (algae) which can suddenly die-off and decompose causing an oxygen shortage.


Reduce nutrient inputs by diverting overland runoff that is rich in nutrients (animal feed lots, crop fields, etc.).  

Dead Vegetation


Low oxygen levels in pond water. Fish found dead and/or gasping for air within a few days after large amounts of aquatic vegetation was treated with herbicide.

Mass die-off of aquatic vegetation from natural causes or herbicide use. Large amounts of rotting vegetation will use up oxygen supply in the pond.

Pond banks should have 3:1 slope to reduce excess vegetation growth. Treat no more than 25% of the pond with herbicide at one time, or use Clear Pond beneficial Bacteria to break down and consume the dead vegetation




Fish die from low oxygen levels. Dead fish seen floating along shoreline soon after ice melts. Few, if any, fish caught in the spring compared to numbers caught the previous season.

Snow covered ice stays on pond for an extended period of time keeping sunlight from reaching plants to produce oxygen.

Shovel snow if it is greater than ~2 inches deep, removing at least 30% of the coverage and/or install aeration system to prevent complete ice cover.

Organic Pollution

Fish die from oxygen shortage. Look for large sources of organic matter which entered the pond, especially after heavy rains.

Excess animal wastes, leaves, decaying vegetation, and other matter consume oxygen as it decays. Large amounts of decomposing matter deplete oxygen supply

Prevent organic matter from entering or building up in pond. Cut trees back away from pond. Divert animal waste runoff around the pond. Use aeration to speed up the decay process and reduce buildup.

Toxic Substances

Fish die from direct exposure to toxic chemicals. May cause complete or partial fish kill depending on the amount of dilution rate as the chemical enters the pond. Toxins will often kill other aquatic life (insects, tadpoles), while oxygen shortages win not.

Pesticides, herbicides, mining wastes, petroleum products, fertilizers, and other toxic chemicals enter the pond via surface runoff from nearby land. Often occurs after heavy rains wash recently applied insecticides or fertilizer into the pond.

Levee the pond or divert runoff originating from potentially toxic sources (crop fields, golf courses, etc.) If possible, avoid using potentially toxic substances with the pond's watershed.

Natural Causes

A few fish found dead along the shoreline in early spring.

After a long stressful winter, a fish's natural resistance to disease is lowest in early spring. Spawning stress may also cause a few fish to die. Larger and older fish seem to be more likely to die of natural causes than smaller fish.

None: let nature take its course.

Information provided by the Ohio Department of Natural Resources.