The Grass carp tolerates some salt, high temperature ranges, and low oxygen. It also survives better than native fish in polluted waters. Grass carp, as you might guess from the name, eat grass and other underwater plants, but will also eat insects and decaying plants.
Because of their amazing ability to eat (40% of their body weight each day), grass carp were brought to the US in 1963 for research on controlling aquatic plants. They weren't carefully monitored and were found in the Missouri and Mississippi rivers in Arkansas by the 1970s. Because they reproduce quickly and eat the plants that other native species depend on, they are considered an invasive fish and are already established in most of the Mississippi River (map).
For more information:
US Geological Survey