The good news is that you can't accidentally overdose your other birds on niacin! Also, the shakey leg can be cured generally. I would get either a human liquid B complex like this and give one dropper full to the affected bird (put it on a treat) twice a day. Don't dilute it in the drinking water. Keep putting the brewers yeast in the food for everyone.
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Or you can get cattle B complex at the farm store. I believe
@Miss Lydia can give you the dosage on that one.
Give your duck swim time and maybe space to practice walking outside of the brooder. His legs will get stronger. I would keep giving the B for at least 2 weeks. You should see some improvement in about a week.