Writing Prompt: Koi Catastrophe

For your story, say your character is in charge of monitoring a koi pond for a special breeding company. Then, suddenly, something goes wrong and the fish start mutating in a strange way. Decide what caused the mutation (perhaps your character will have to figure it out if it was foul play, or decide how to tell his boss if he thinks he dropped that vial of experimental formula into the tank by mistake), what the mutation is, and how your character is going to resolve the conflict.