LOL, if you think they are happy with regular food, try leftover cooked broccoli in a dish full of water. Must be a good substitued for duckweed, 'cause mine always loved it!
Word of wisdom--if you EVER hear a quack from a muscovy, do not walk, RUN to the pen with a big stick or a gun. The only time a muscovy hen quacks is if she is in danger of dying. The one and only time I heard a quack from one of my girls I ran like H3ll--a dog was in the pen and by the time I got out there it had her pinned and was about to kill her. (I ran it off and she was unharmed).
Muscovies breed like rabbits, so unless you are willing to eat the offspring, stick with just one gender. I know of 2 cases where muscovy hens successfully hatched 22 ducklings apiece, and had one that was sitting on 24 before a possum got in the coop and ate the eggs. They'll set 2 or more times per year and are outstanding parents--my drake would bully any adult duck that got between him and food, but would back away from the feeder while the ducklings were gobbling; he also goosed cats that got too close to the brood and would come running if a duckling peeped in distress.
I don't have any currently, but if I had ducks again, it would be a group of muscovy hens. They are such a big, calm quiet breed that I just enjoyed having them around.