In short, the fish is a monster of an invasive species and completely blows apart the food web wherever it shows up. It's been making its way upriver from the Gulf of Mexico seeking cooler water. The fear is that it will find a route into the Great Lakes and wipe out, well, just about everything.
It's that serious. This fish is baaaad news.
Sadly, it looks like our best efforts have been too late for at least one of the lakes, the U.S. Army Corps of Engineers has found evidence that the carp is in Lake Michigan. Their announcement came on the heels of a U.S. Supreme Court decision that allowed Illinois to bypass taking more action to keep the fish out of the lake.
Here's a good video explaining the myriad of dangers created by the fish.