my pond is about an acre. I would say over the last couple years it was 90-95% pondweed. Pond filled up about 4-5 years ago and then the pondweed came in. Has been great for my fish but unfortunately it is kinda hard to fish a pond full of pondweed. You have a good sized body of water. I would think you would need more than 9 per acre, but I am no expert.