Sergei considers the goldfish to be his friend. From the statement, it
is shown that Sergei has trust issues with other beings (May it be a human or
animal). He may have a dreadful experience in the past that prevents him from
befriending others. And he made an exception with the goldfish because he found
out that the goldfish will not leave him even if he is troubled.
The BEST way to organize the information on a poster will a a timeline.
Explanation: I say a timeline because It is a easy way to use numbers and dates on a timeline and a timeline is a display of a list of events in chronological order.