thank you very much for all your replies. no other ducklings have gone down yet, touch wood.
the little week old has now died so thats 4 dead and one survivor.
totally baffled
the chick crumb is all I feed, none of the chicks or ducklings get anything else, just chick crumb & water.
the fancy feed chick crumb I am using is suitable for chicks, ducklings & goslings and is medication free.
wish there were other clues but I really am at a loss.
fingers crossed for the remaining ducklings