Tom,
I haven't read every post so I apologize if I'm repeating others ideas. As opposed to fertilizing your pond, which if done incorrectly, untimely, or at the incorrect rates will cause more headache, why not try a more eye appealing aquatic dye like aquashade. It may not completetly prevent your problem but it could slow it down. Give it a good dose every other month to maintain a good shade and it'll block out some light without adding more nutrients like fertilizers do. If your pond is close to your house, are you fertilizing your yard? That would be a problem if you're receiving runoff from the yard. This is only my two cents, but I wouldn't count on the grass carp for much. Best of luck.
Mike
