Carp were introduced into North America when the native fish numbers were greatly diminished through commercial fishing. However, they took on the "trash fish" monniker soon afterward because the rivers into which they were dumped (around the Great Lakes)were so horribly polluted, and the fish eat vegetation and do some scrounging around in the mud.
By polluted, I mean the Ohio river had actually caught fire.
Anyway, carp are enjoyed all over the world as a good meat fish, easy to raise and filling. Today, most of our rivers aren't nearly as polluted, but if you find some in some good, clear water (like, say the Feather River in California), you can get some good-tasting fish.
Tilapia is darn-near the same thing.