We have found the best thing for when they already are having trouble is liquid BComplex. https://www.backyardchickens.com/threads/niacin-deficiency-in-waterfowl.1367557/
I read on the goose thread a few days ago that if you buy reg niacin in the capsule at any pharmacy and open it up you have regulat niacin. so go to any grocery store dollar store or drug store and buy niacin even slow release and open up the caopsule and pour int their drinking water...
Lil English peas[green] they are round and can be found in the frozen veggie section at the grocery, just bring them home and thaw some in warm water or toss some frozen into their swimming pool if older ducks. if young ducklings then smashing them a bit will help them to be able to eat them.