Great that you are on top of things. Yay!
There was a study published about how niacin deficiency is not a problem for ducklings, but I declare, I have seen so many forum members improve duckling problems with added niacin, I don't know what to say, except that my personal theory (guess) is...
Some people say they see a difference within a few days.
Give them some swim time, too. Supervised, in warm water, but it gets weight off the legs, improves circulation and strengthens muscles.