@JuanG , I was saddened to read about what happened to your ducklings. As I (and others on this site) have learned, farm/feed store employees are not always knowledgeable about animal care, so it's critically important to educate yourself about the animals you're acquiring. If you haven't already seen the articles in the Learning Center, there are resources here:
https://www.backyardchickens.com/a/other-backyard-fowl-ducks-quail-turkeys-geese-etc. I also like to keep a book or two on hand, like Storey's Guide to Raising Ducks. It's a comprehensive reference that will come in handy.
I raise ducks, and will typically feed chick starter, which has a fairly high level of protein in it (usually around 21%) for the first two weeks, supplemented with brewer's yeast for the niacin - ducks have higher niacin needs than chickens, and if they don't get enough, they can start to show physical symptoms like leg problems. At the two week mark, I reduce the level of protein to around 16% to prevent a wing development problem called "angel wing"; this is a mostly cosmetic problem, but it can prevent a duck from flying. There are different theories about what causes angel wing, but high protein levels in feed seem to be a factor.
Ducks have different nutritional needs throughout the different phases of their lives, depending on whether they're female (ducks) or male (drakes). A laying duck, for example, will need a supplemental calcium source to ensure that she has sufficient calcium for egg formation or she can experience egglaying problems and health issues. The Learning Center articles (and the Storey's Guide, if you choose to go that route) expand on these and other aspects of raising ducks.
Best to you in your learning journey.