Very healthy duck got sick suddenly. I dont know what to do. Pls help!

[Isaac 0]

We supply them with both a multivitamin syrup, another amino acid solution for animals that is high in niacin and vitamin D. Although we were mainly giving it to Bambi since Bobby was healthy and ate a lot. He still got some but it was around 1/3 of what Bambi gets because we were not sure about the effects of the particular products we used. We were comfortable getting little experimental when it came to Bambi because of our desperation however we didn't want to take the same risk with Bobby, starting from today we will up Bobby's supplement doses as well. We just want to know if this can be something else since it developed so rapidly for Bobby. He went from healthiest duck to not being able to stand up in a single day. We are also worried he catched a cold.

Would you mind posting the brand of the vitamin you're using, and tell use what dose you've been giving it to them at, and how? Does your feed bag have a niacin content %, if yes, could you tell us?