While there are medicated feeds that have proven safe for ducks, there are others that have not. I prefer to not even worry about it and feed non medicated so there's no risk of it bothering them. John Metzer has a very good article on the subject posted on his website. I would encourage you to peruse all his articles as they are gold mine of information to those interested in raising waterfowl properly.
Most people don't recommend using chick starter for duckling as its lacking the extra vitamins / minerals that they need. Niacin (vitamin B3) is the most essential but I'm told there are others as well. If you can only find chick starter in your area then its highly recommended to use a good vitamin / mineral supplement, or at least to supplement Niacin.
This is the best one I have found, it is also recommended and sold by Metzer Farms in their feed/supplement section. https://www.qcsupply.com/549992-bvs-starter-pak.html
Purina Flock Raiser is what the majority of us have found best suits raising ducklings, but there are others as well. Once your ducklings get bigger you can switch to pellets if you like. Purina flock raiser is available in a pellet form, but for some reason its hard to find. Only one feed store within driving distance of me sells both the pelleted and crumble forms.
Supplementing Niacin is generally not needed with Flock Raiser, however it is never a bad idea as you can't overdose it if your giving it at the suggested amounts. This is especially true for Pekins, as they are the fastest growing duck and therefore need the most nutrients to support their fast growth.
Nutrena All Flock which is about the same thing except its only available as a pellet, and Nutrena does not recommend feeding it until your birds are at least 8 weeks old. I have wondered if this means there isn't enough Niacin in it for the ducks fast growth stage (which is the first 8 weeks of their life) or if they are simply concerned about the size of the pellet as it is a very large pellet.
I hope this answers your questions about the proper feeding of ducklings. If not, continue to ask and we will do our best to answer you.
I highly encourage you to buy and read Stories guide to raising ducks, by Dave Holderead (hope I spelled his name right). Dave and John Metzer are the best sources of information I have ever found and I believe them to be the two most knowledgeable people on the subject in the world.
