Fish Kills – The Causes and Solution

For a pond or lake owner, there is nothing more terrible than having a sudden mass extinction of aquatic life-otherwise known as a Fish Kill.

Fish kills come in many shapes and forms-sometimes big fish die, sometimes small fish die, other times all fish meet the same demise. Some fish kills are limited to a particularly sensitive species of fish, others do not discriminate and affect all fish (including the big game fish you keep fishing for).

A fish kill can be as small as 10-20 fish or as large as 10-20,000-and once fish begin to die, it’s generally too late to remedy the situation. The only way to thwart the fish kill phenomenon is prevention. Before we can prevent fish kills, we must understand the six possible causes behind them:

1. Oxygen Depletion

Oxygen depletion is the most common cause of fish kills. Oxygen must be present in the water for fish to breathe-an oxygen level of 4-5 parts per million is recommended for healthy fish.

When oxygen levels dip down to 2-3ppm, fish become stressed. Fish swimming near the surface, sometimes gulping the air, is a common symptom of this stage.

Large fish are the first to die in an oxygen depletion fish kill, followed by the smaller fish. Fish kills of this sort often happen overnight or in the early morning-and typically occur in a matter of hours.

2. Large Plant or Algae Die-Off

When a body of water rife with plants and algae suddenly receives less sunlight or receives algaecides and herbicides, large die-offs can occur. The resulting decaying matter wicks oxygen out of the water, suffocating your fish.

As a rule of thumb, if your pond has more than a 20% covering of algae or surface plants, avoid algaecides and herbicides and get oxygen into the water as quickly as possible (see solutions).

3. Turnover

Stagnant water tends to migrate into distinct layers-like a layer cake. Fish typically reside in the layers near the surface where they enjoy warmer water and higher dissolved oxygen.

The layer at the bottom is like a fish no-man’s-land as it is generally is colder, has little or no oxygen, and is filled with accumulating toxic gasses that are trapped by the layer differences.

In the spring and fall (and sometimes more often if the weather conditions are just right) these layers mix, sucking all the oxygen out of the water and introducing toxic gasses to the fish. If severe enough, turnover can easily cause a mass fish kill.

4. Surface Freeze

When the surface freezes, the ice acts just like the temperature difference mentioned in #3. Gasses poisonous to the fish build up, and can sometimes wipe out entire populations.

5. Toxins and Chemicals

If pesticides or other harmful chemicals run into your pond, it can obviously have a damaging affect on any life within it.

Sometimes the application of herbicides or algaecides can lead to fish kills, as the chemicals is lethal to all sorts of aquatic life.

6. Disease

Disease affects all life forms at some point or another, and it’s hard to prevent. The symptoms of disease, however, are often less severe than most fish kills and happen gradually over time (rather than over the course of a few hours, like a normal fish kill).

Disease generally affects only one species of fish, seldom killing all of them, and rarely do they occur on a mass fish-kill scale.


Add oxygen to the water keeping the water fresh and fish happy

Mix the layers to expand the fish habitat and break up toxic gasses

Prevent excess Algae from taking over and wicking out oxygen

Avoid the Application of Chemicals that can harm fish

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