|The common carp (Cyprinus carpio) is now found all over the world|
|Ornamental koi carp in numerous varieties|
The major reasons why this species has become invasive include a variety of things: altering habitat and thus changing ecosystems, affecting the normal nutrient cycling of a habitat and disturbing bottom dwelling species, reducing local species diversity, competing with and threatening native species, and loosening sediment, thus clouding water among other things.
The current uses for the common carp include cultivation for food (aquaculture), as an aquarium species, and also for various forms of fishing. This, interestingly enough, includes both commercial and sport fishing.
The common carp is on the list of 100 World's Worst Invasives at #30.
Images are from Wikimedia Commons under Creative Commons licenses: one, two.