I was participating in a stream survey a couple of years ago in Chicagoland and one of our leading resource people pulled a roundish, lumpy green fish from the seine. He asked us what it was. Following an embarrassed silence, he grinned and noted that it was a goldfish turned wild colored. It was pretty ugly. It doesn't take colored carps (or the surviving colored carps) long to assume wild and more cryptic colors. Golden sports are not uncommon among a number of aquarium fish. They are easier targets for predators though.

The other carps are a bigger problem than goldfish, but most of them eat eggs and fry of other fish, stir up the bottom - covering and suffocating eggs, eat and uproot plants and consume other foods which might have served the natives. If a new fish fits into the habitat, it replaces something in part or in whole.

I first ran across mention of exotic fishes in Oz in Allen's TFH book Fishes of Australia. A list of Australian fishes http://www.nativefish.asn.au/taxonomy.html
lists everything which they have records of in Australian freshwater.

A lot of the Australian freshwater fishes are descended from marine fishes. I guess that is what happens when continental drift runs a continent under the south Pole, killing whatever was there previously.

I'd also forgotten some of the wonderful names of Australian natives. I.E. Mouth Almighty for a freshwater cardinalfish, Gulf Saratoga and Saratoga for their two Asian arowanas (Scleropages jardinii and S. leichardti), Sleepy Cod, Strawman or Blackmast, Murray Cod, Trout Cod, Freshwater Cobbler, Honey Blue-eye, Bullrout (a scorpionfish), Barramundi, and all sorts of grunters.

Exotics include at least four cichlids, weather loaches, Cyprinids such as the Goldfish (found in seven states), European Carp, Roach, Tench (all European pond fish) and the aquarium Rosy Barb. Seven livebearers (the Eastern U.S. Gambusia, holbrooki, Gambusia dominicensis, Phalloceros caudimaculatus (from SE Brazil, Uruguay, Paraguay and maybe south of there ), Sailfin Molly, Guppy, Swordtail and Platy. There is even a killie, the semi-tropical American Flagfish. The first three livebearers and sometimes guppies may have been introduced for mosquito control. The others were probably aquarium releases.