AL, how many grass carp do you have, to control the grass? 4ft deep will induced plenty of sunlight, which enhance the eel grass to be prolific. Very prolific. When the water is clear, you'll have a greater chance of it invading other part of the pond very quickly. Natural method is best.
Chemicals induction is not the way to go to keep plants in check if you can help it. Best to stick with plants that can be controlled by active feeders. It's a pain when birds bring in invasive species that you don't want, and immediate attacks are needed. But once again, best to stick with plants that are beneficial and controlled by natural predation.
