In my opinion Grass Carp might be the answer. I had an eel grass that I had planted in my pond and found it coming up in bunches and finally eliminated. Sago might be something Grass Carp like being similar to grass and easy to eat. It dawned on me one day that there is a reason that Grass Carp are called Grass Carp.

You pond being a third of an acre I'm not sure you would want more then three GC in it as they might take out more then the sago. Around my area you usually have to buy six GC. I suppose you could do that and then try to kill some if they were too many.