Sergei doesn't like people knocking on his door because he had bad experiences with the KGB when he was younger. This is why he's so mad when Yoni comes in
Sergei is so protective of his fish that when he thinks Yoni is trying to steal it, he kllls Yoni without even thinking about it..
Sergei talks to his fish about the murder, and how he did it to protect the fish, which is his friend. The fish talks back to Sergei, and tells his that he should use the last wish to save Yoni. Between the paranoia, confused thinking, bizzare actions, and hallicinations, Sergei is diagnosed with Schizophrenia.