Having been there done that and given up. I too would be interested too in a successful treatment.
On the copper sulfate.......... I thought that just colored the water so algea didn't get the light it needed to grow.
And tell more about the specifics of treating- how much and how do you apply?
Heres what I have learned-
Killing it off is easy, but it comes right back(reseeds itself). I tried killing it off for three years, the fourth year I skipped and its right back where it started. I even drained the pond and left it dry the whole summer the first year. I think my main problem is that I have trees around three sides of the pond(bought it that way) and too many nutrients are getting into the pond.
I may take one more shot at it next year. I am going to kill it off and have 20-30 carp ready to go in(have a buddy that says he can net them for me). Hopefully when it reseeds the carp can eat it fast enough to keep it from taking the pond.
Believe it or not there is a market for duckweed, idiots buy it by the lb to feed to Koi Fish. I have not yet found a dealer that doesn't have it own plentiful supply though
I have thought about taking a sample and having it tested for its feed value. It may be worth harvesting