Various products like vinegar and hydrogen peroxide are good on terrestrial weeds, but who knows what they'd do and how much they'd change the pH of the water. I did a quick survey of the non-organic treatments. They sound awful - lethal to any wildlife in most cases. Sounds like it's time to get the pool scoop and find a place to plop the duckweed to dry. Maybe out of the water it is more useful? As a food or fuel?