This is from the Texas A&M website.

"Grass carp will seldom control aquatic vegetation the first year they are stocked. They will consume coontail. Grass carp stocking rates to control coontail are usually in the range of 7 to 15 per surface acre."

I would think that you need to stock on the high side of that recommendation. With that much coverage it may take a few years to clean most of it out.