What killed the fish? How do I clean them up? How do I prevent fish kills?

Aquatic professionals will get these questions a lot during their career. There are generally four causes of fish kills —both separate and related —including oxygen depletion, algal blooms, toxicity, and disease. The solutions are described below as we discuss Naturalake Biosciences.

Oxygen Depletion

Fish need oxygen. Generally, they need more than 3ppm of dissolved oxygen in the water to survive. Oxygen depletion is by far the most common cause of a fish kill and it can happen in cold or warm water environments. The underlying cause of the oxygen depletion can be from one or a combination of many factors.

In warm water, one common cause of oxygen depletion is through a large rain event or even abnormally high winds that break the natural stratification of a lake or pond. The oxygen rich warm water on top is quickly mixed with the cooler water lacking in oxygen on the bottom and results in a thinning of the oxygen supply at all levels. Another common cause of oxygen depletion in warm water is the increased biological activity. High nutrient levels and extended periods of sunlight lead to increased aquatic plant and algal growth. The algae blooms during the day and dies off at night creating more demand for oxygen. The microbial community in the muck on the bottom of the lake or pond also becomes more active consuming all the dead organic matter.

In cold water, the most common cause of oxygen depletion is a combination of decreased water volume, increased ice thickness or snow cover, and an abundance of decaying organic matter. In other words: more organisms are confined to a smaller area with a more finite amount of dissolved oxygen. Aquatic plants and algae cannot provide oxygen to the water body when photosynthesis stops due to lack of sunlight. Muck buildup from runoff, fish waste, and dead aquatic vegetation is decomposed by the microbial community, using up valuable oxygen. Overall, the aquatic environment is depleted of oxygen and the fish die off.

Algal Blooms

As mentioned earlier, intense algal blooms during the day result in a rapid death of algae at night. When the algae die, they can create and intense oxygen demand in the lake or pond.

Harmful Algae Blooms – Blue-green algae, also referred to as cyanobacteria, can cause fish kills as well as illness and death in animals and humans. These organisms are capable of producing neurotoxins, hepatotoxins, and dermatotoxins as part of their metabolic processes. These toxins, when released in the water, not only kill the fish but can accumulate in the tissue and internal organs of fish making them dangerous for animals and humans to consume. The toxins may also persist in the water column long after the bloom is gone.


Microcystis in a Texas fish farm


Toxicity from pesticides, acids, fertilizers, or pollution can come from the mismanagement or overuse of herbicides and algaecides, agricultural runoff, and leaking septic systems. The toxic compounds can impact the entire population of fish or just a specific species or size. Once toxic compounds enter a lake or pond they can be very difficult to remove.


Many diseases or parasites can result in fish kills. Viruses, bacteria, and parasites are naturally occurring in lakes and ponds. Typically, fish are more susceptible to disease or parasites when they are already stressed from other environmental impacts such as poor water quality. In fish farms, disease spreads quickly with dense fish populations, excess muck building up, and high ammonia concentrations.

The Best Offense is a Good Defense

Naturalake Biosciences recommends taking a proactive preventative approach to limit the probability of a fish kill occurring. Often, a good first step is consulting with an aquatic professional to see if aeration is appropriate for your lake or pond application. Aeration can provide extra dissolved oxygen to a water body and ensure it is adequately mixed to prevent oxygen depletion. In addition, Naturalake Biosciences recommends supplementing lakes and ponds with probiotics to combat external nutrient loading, prevent organic muck buildup, and degrade toxic compounds as they enter the water. MD Pellets, Temperature Driven Solutions, and Water Column Clarifier can all be great options for a proactive treatment program.

When a lake or pond is already experiencing fish kills, Naturalake Biosciences recommends contacting an aquatic professional to help diagnose the cause of the fish kills. If the lake or pond is experiencing excessive algal or plant growth, Naturalake Biosciences recommends using a biocatalyst such as AquaSticker or PondZilla with the appropriate algaecide or herbicide. Using a biocatalyst will increase the efficiency of the chemicals and ensure the vegetation is fully degraded so it does not contribute to the muck layer or internal nutrient reservoir. Additionally, probiotic products such as MD Pellets, Water Column Clarifier, or the appropriate Temperature Driven Solution can be used to cycle out nitrogen and phosphorus compounds, degrade the organic muck layer, and aid in the breakdown of toxic compounds.

For an existing lake or pond that has a fish kill, tank mix our Temperature Driven Solution and spray over the dead fish. The fish will degrade quickly and odors will disappear.

Learn more about our Temperature Driven Solutions:

Naturalake Biosciences Temperature Driven Solutions are dry probiotics that cycle out nitrogen and phosphorus compounds. They are independently optimized to perform according to water temperature.

Polar Blend

Summer Slam

Other Products for Fish Kill Prevention:

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