Excess Fertilizer added to the farmland, so when it rained the fertilizer overflowed the farmland and created runoff rivers.
The fertilizer runoff flows into a larger body of water creating an overgrowth in aquatic plants and algae
However, because of the overgrowth, sunlight wasn't able to reach the the bottom of the body of water. This caused algae to die so they had to be decomposed but, since plants couldn't grow due to lack of sunlight, there was little oxygen. When the bacteria decomposed, it took up all the oxygen thus making the lake anoxic causing other organisms, like us fishies to die.