Don,
I sure can't answer your question, but here is a little info I found from Texas A & M site:
Sago pondweed can be removed by raking or seining it from the pond but will reestablish from any remaining roots and seeds.
Fertilization to produce a phytoplankton or algal �bloom� prevents the establishment of most bottom rooted aquatic weeds and produces a strong food chain to the pond fish.
http://aquaplant.tamu.edu/management-options/sago-pondweed/
Gary