Introduced species
One introduced species is the tilapia.  It is a fish that comes from warm freshwater areas of Africa, North and South America, India and Sri Lanka.  It has been taken to other areas because it is good for fish farming, and it also is a good pet for an aquarium.  When the fish is released into other parts of the world, it quickly takes over from the native fish.  It can do this because it breeds quickly, eats all the available food, and eats the eggs and babies of native fish.  This can cause the water to become so muddy, that the native fish cannot live.  Tilapia has been listed as one of the 100 worst introduced species.  A person owning or selling one in Australia could be fined up to AU$11,000.