If I remeber correctly, grass carp, as their name implies, are primarily vegitable eaters. They're put into ponds and whatnot to keep vegitation down - but sometimes can go too far. I seem to remember something about only stocking one sex to avoid proliferation.
I think they're more difficult to catch than common carp, but I seem to remember someone posting about fishing for them at a pond after the mowers went through sending a bunch of clippings into the water. He tied a grass blade to a flyfishing hook and actually had some luck apparently. I cant vouche for this being something other than an april fools joke though. I've also heard of luck using any local berry or fruit hanging over a body of water - like a mulberry.
I do know they get huge. Good luck.