Martin was a one-episode-only character, as Michelle's pet goldfish.
His appearance was in the episode "A Fish Called Martin" from season four. She wins him at a carnival, and Danny states that it is very expensive for her to do so. He only lives for a short time. When Joey tells her to keep him clean, she misunderstands and she gives him a bubble bath in the tub, which kills him. Later in the episode, he is replaced by Freddy.