most people don't think it out, and think that they can just grow duckweed to feed their fish, and have a "zero input" system.. but they soon find that doesn't work..you need input!
i've looked up lots of info on duckweed, and for me it has it's place..
i have three 55 gallon glass tanks, one 45, two 20's and 3 tens..
in all my glass tanks,(besides sponge filters) i use duckweed to consume ammonia produced by crayfish (i've got a couple other plants in the tanks as well, not only to consume ammonia and nitrites, but as habitat for scuds crayfish and minnows), which are also an excellent supplementary fry/minnow feed by the way), and as it grows i thin out each tank every couple weeks, but basically every weekend i put several net "scoops" into the fish tank... the tilapia clean it up over night..
but my primary feed is a mix of several varieties of commercial fish food (low to high protein)..
if i was using raft tanks, i'd be concerned about mosquitos, and would populate the troughs with minnows to eat the larvae..the problem there is that the minnows will also eat the gammarus/scuds..
in the end, it's all about balance