This is an old thread but since I'm new here I would like to comment. I manage 7 trout ponds that are infested with eurasian water milfoil. We stocked grass carp in an effort to control the milfoil many year ago. As has been stated they don't eat milfoil. Somehow the expected lifespan of the grass carp has been exceeded by at least double and we have these huge carp swimming with our trout. It also appears that we got at least one batch of "guaranteed triploid carp" that reproduced because we have small carp, too. We wasted our money due to bad advice to stock carp and now have degraded our fishery as well.
Larry