Duckweed and goldfish
How are goldfish able to kill off duckweed? Wouldn't the duckweed kill off the goldfish due to the lack of oxygen and nutrients in the pond? What is the biological makeup of duckweed? What's your background surrounding duckweed? Can duckweed populate a bigger body of freshwater such as a lake (ex: Gull Lake in Brainerd, Minnesota)? If so, at what level does it populate the water? What effects does it have on the ecosystem there and at what level?
Goldfish, Koi and grass Carp can help with pond Duckweed, but will not eliminate it completely.
Our expertise is more geared to garden ponds, a much smaller scale then lakes.
I would suggest contacting the MN DNR for specific information on your questions.
I have listed several links for you as the very matter of aquatic weeds is well studied in the great state of MN!