The Invisible goldfish was a dead goldfish brought back to life during Victor's second experiment. An unknown variable caused it to be invisible.
Trivia
The Invisible Goldfish is based of Universal movie: The Inivisible Man.
It is unknown what happen to the Invisible Goldfish or if it's still alive.
The Invisible Goldfish was going to reappear in the scene of the monsters attacking New Holland. But Tim Burton cut the fish out of the scene because it took up too much time.
The Invisible Goldfish isn't mentioned in the end credits.
Fan theory is that the goldfish was turned invisibee when the lightning hit a mirror during the experiment.