Welcome to BYC. I'm not a duck person but I do know that your set up is not conducive to good duck health. I will leave the advice to @Jenbirdee
Good luck to you and your flock!
We just got two ducks this spring and fed them what the chickens get, Kalmbach's Flock Maker once I learned it is complete for ducks too.
After a couple of months go by, they started limping though, one really bad. I had learned from people on BYC, mostly @Miss Lydia and @Jenbirdee that ducks may need to be supplemented with niacin as their legs are usually the first to show the deficiency. I got some nutritional yeast and frozen peas. Between the two, that fixed it!