When collected fresh, approx. 90+% water content (a non-nutrient), high in carbs, ash, and fiber, the latter being what Duane was referring to. As far as actual nutrient content, including protein/fat/minerals etc, that would be dependent on the water they are grown out in, so that will obviously vary.
But I agree, all fish, even those that are classified as carnivorous, should be consuming some aquatic plant matter on a regular basis. I supplement mine with dry aquatic based commercial food, I would personally never introduce duckweed to a tank due to the mess it can make if not kept under control. That, and the nutrient levels are a lot higher when aquatic plant matter is fed on a dry matter basis, vs fresh.
For reference:
www.feedipedia.org