Purposeful Fishes

Herself is looking after Cherished Friend's fish for a few days. It cheers her to see the fish swimming about; they are small, colorful, and purposeful. There is one petulant piscine who hides just long enough for Herself to begin to worry that it has croaked, before it makes its appearance - ta-daaa. Can a fish be mischevious? What goes on inside those teeny tiny fishy brains?  I wonder.