Remove the surviving chicks to a different pen away from all ur adult birds, then call your local or state Extension office and get a Necropsy on the dead birds if you still have them
Ducks need more of certain minerals that are not in a layer feed if they don't get them they develop leg problems also all chickens/ducks can have oyster shell to prepare them for laying and also to support healthy bone structure in ducks adding brewers yeast to duck feed Is beneficial