NutriDrench will not fix a niacin deficiency, you really need that liquid B Complex posted last page.
My TSCs, both, had NO idea what I was talking about and had never heard of ducks needing niacin. They insisted nothing in store was a niacin or B supplement (niacin is a B vitamin, in case you didn't know.)
Ignore the employees, ask directly for Durvet cattle injectable B Complex.
Amazon is also a good route, but it'll take ya two days+.
Grab yourself a thermometer to put in your brooder. A few dollars really well spent! Took me three Brooders to figure this out.
Ducklings appear to be more heat sensitive than chicks, from my experience. They could very well be too warm, while chicks are happy. Too hot ducks could result in a slight dehydration (losing vitamins!) And twitchy wings to relieve some heat.