David, 20 pounds of tilapia is about half of the suggested stocking rate for 2ac in your area, so I would not expect the T to keep up.
What other fish have you stocked?
Any LMB to munch on the Tilapia at the end of the year? If so, I would stock 60-80 pounds of tilapia next year, or get approved for Grass carp, and not fertilize until the current plants are controlled. There is always the chemical route, but that is expensive and when the pondweed decays, it will likely come back as more FA.
