Duckweed is likely to be hit or miss.

Maybe grazing fish, ducks, unsuitable water quality may subdue or eliminate it.

Or maybe the pond is over fertile, there is no significant grazing and it takes off and smothers the entire pond. Its high growth rate producing huge amounts of rotting debris, polluting water quality and blotting out all light to kill all other beneficial plants, insects and fish

Not a pretty sight, a pond or lake destroyed by invasive plants

Wouldn't risk it, except on a small pond which is fairly easy to keep it thinned out by manual clearing

Regards, andy
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