How do I prevent grass growing in our wildlife pond?

Question:We have a 40ft x 50ft long 4ft deep unlined wildlife pond and the grass is growing in it. We have 6 grass carp currently in the pond but they don't appear to be making much difference. My parents are worried that the pond will eventually fill in and become more of a bog. Will this happen and if so is there anyway we can prevent this other than pulling the grass out by hand or machine?

Answers:
I would call your county extension agent for solutions. An herbicide may be effective, but I no longer live in the country. It would take a lot more than 6 carp to make a dent.

Soil washing into the pond from rain runoff would eventually fill it in, but grass, cattails and wildflowers will just make a marsh.