Though there is one fatal flaw to this story: it was never declared that the unknown pet in the last deduction is a fish!
Nonetheless, these are logic puzzles, and none should have trick answers, or unwarranted assumptions. Has no one done these before? Here's another classic:
A farmer has a fox, goose and a bag of grain, and one boat to cross a stream, which is only big enough to take one of the three across with him at a time. If left alone together, the fox would eat the goose and the goose would eat the grain. How can the farmer get all three across the stream?