Ducklings don't need the same medicated chick starter that chicks do because they are not susceptible to the same problem as babies.
However, as they continue to mature, I believe one of the things to be aware of is that the ducks should not drink so often from the water that they swim in, which ultimately means they pooped in it, too. This might not be too much problem for large pools or ponds, but if you are giving them only a small tub or even kiddie pool, be sure to clean it out often.
Decide if you want to let the ducks have enough space for swimming in or just for dunking their heads. The latter should help with not drinking water full of their feces but at the same time, it seems to be denying them some prime swimming time where you can see them having fun and enjoying the water. So take your pick.