Episode 36 (Death of a Goldfish)
A fish in Rogers' tank has died. Rogers buries it and, with Bob Trow's help, constructs a tombstone with a pentagon that bears a fish drawing. In the Neighborhood of Make-Believe, concern hits when the Trolley stops at the Castle and doesn't move. Eventually, with help from Bill Platypus, they find out that the Trolley simply suffered a derailment, preventing full electrical pickup (as explained earlier by Mister Rogers).
Aired on March 23, 1970.