There has been some attention paid to the discovery of Lionfish in Caribbean/Southeastern US waters. Most likely the result of discarded aquarium fish, these beautiful but voracious reef predators have drawn concern because of the potential impact they can have on the reef community. Research is ongoing to monitor their impact and while it's known that they can prey heavily on smaller reef fish, there has been some initial evidence that the lionfish themselves have fallen prey to large groupers. Whether this will ultimately balance out the situation remains to be seen.
Invasive alien species (IAS) can take the form of plant or animal - from seaweeds and exotic forms of algae to invertebrates to fish, and more. Over 200 fish species have been introduced into the US alone, often through the home aquarium trade where unwanted fish or plants are discarded into lakes, rivers, streams, or directly to sea. Our increasing globalization and increased shipping has also been a major contributor, with ships having the ability to inadvertently transport a variety of species of seaweeds or invertebrates like zebra mussels that can push out endemic species or clog pipes and other man-made waterways.
Scientific theory used to adhere to the idea that a rich biodiversity was the best defense against IAS. Now, there are differing theories based on an ecosystem's species' maximum utilization of resources which allows for little variability. Splitting hairs? Might seem that way, but the bottom line is: the best defense is a good offense - healthy marine ecosystems and vigilance on our parts regarding international regulations and inspections. And if you are a home aquarium enthusiast - a little common sense.