There are many natural and manmade causes that can lead to fish kills. Natural causes may include changes in weather, lake water depth, cloudy or muddy water, and fish community structure. High levels of algae growth can rapidly change the level of oxygen in a lake or produce toxins that result in fish kills. Algal blooms can be caused by high levels of nutrients entering a lake via runoff from fertilizers, septic tanks, pet waste and yard waste. Misuse of pesticides or herbicides around a water body can also cause fish kills.