No pet animal should ever be dumped into the wild...be it a dog, cat, snake, fish, bird, ferret, what ever.
The Florida Everglades is now being over run by various species of Python, with the Articulated Python (which can reach lengths of 20 feet) having established a breeding population estimated to be in the 150,000 range.
The Snakehead, a very aggressive predatory fish from Asia, has established breeding populations in a number of water ways, including the Patomic River...which connects to many other water ways. All species of Snakehead are on a Federal Ban list, and can not be imported into this country in any form.
The Pacu, a South American fish related to the Pirahna and Silver Dollar, has been found in a number of water ways, but no indication that it had established a breeding population. The Pacu is a large (up to 30 inches for the Red Belly and 40 inches for the Black Belly) powerful fish, but it is a big gentle giant...it eats plants, fruit and nuts. This fish is sold at a number of national chain pet stores (and Walmart quite often) and is totally misrepresented in how large it can get. Most stores I have seen it in, state that this fish only grows to 10 inches and can be kept in a 30 gallon aquarium.....the truth of the matter is that a single Red Belly Pacu requires a 300 to 500 gallon tank with very thick walls as the Pacu has been known to hit the glass with enough force to bust the aquarium. And no, fish do not grow to the size of their tank...they grow to the size their genetics tell them to grow to.
Many ponds and lakes have been overrun with Gold Fish and Koy that have been released by people who no longer wanted them or who found that the cute little fish in the pet store grew far larger than they were told it would grow to. Common and Comet varieties of Gold Fish can, and will, reach 18 to 24 inches in length and can live upwards of 40 plus years....if properly housed and cared for. Koy can reach lengths of 4 feet and there is some indication that a 75 year life span is possible for Koy. Gold Fish and Koy, under the right conditions, can breed like crazy and quickly overstock a pond or lake.
Some states and cities have banned the possession of ferrets due to some being found in the wild after being dumped by owners who no longer wanted to care for them. There is no indication that they have established breeding populations anywhere in the US, but that has not prevented them from being banned.
OBIO