If you have a duckling that *needs* it, you won’t get enough in via drinking water.
It should go in the food, really. I would put 1cc per duckling into an amount of food they’re going to eat today, and do that again at night. Twice a day. The others will poop out the excess vitamins. Do this...
B vitamins. Not just niacin. You can use human b complex tablets or liquid.
once she gets better, I prefer nutritional yeast over brewers yeast for vitamin supplementation regularly.