Gold fish are some of the easiest pets to keep. They need relatively little attention and can live for decades, as long you sprinkle them with food and clean their tanks regularly. Some people, however, decide to release their goldfish into the natural water environment of lakes or ponds. The result is their former pets turn into ‘monster’ fish. Researchers warn that the large invasive gold fish can outcompete local fish, while their feces also seems to disrupt the pond and lake ecosystems.