Here are the three options I always tell people buying ducklings from me, which are the same three suggestions from Holderread Waterfowl:
* Purina Flock Raiser or duck feed--these are the only feeds I know of that are specially formulated for ducklings. Even feeds that claim to be for all young poultry (even if they list ducks among them) are usually deficient in niacin, which causes leg weakness.
* Chick starter PLUS a niacin supplement, either in the form of brewer's yeast on the food, or pharmacy niacin gel caps broken into their water.
* Chick starter mixed with game bird starter. Game bird starter has plenty of niacin, but the protein percentage is too high. Chick starter has lower protein, but not enough niacin. Mix the two, and you have a perfectly adequate duckling feed. This is what I feed my ducklings.
I always avoid medication--why complicate matters? Most places that carry medicated also carry unmedicated, so I just get the unmedicated kind and don't have to worry about whether it's the right kind of medication or not.