I've been reading through my duck books, and here are some other possibilities other than nutritional deficiency:
Medication poisoning- use unmedicated feed
Some kind of congenital neurological defect causing seizures. Not much to do about this one
Duck Viral Hepatitis: Occurs in ducklings under two months. Infectious, transmitted through ingestion of fecal matter. Usually fatal to young ducklings, but death is within an hour of symptoms, so that doesn't match your case. One symptom is seizures as you describe. From David Holderread: "...ducklings that retract their heads over their backs, fall on their sides, and paddle their feet." No treatment. Ducklings may recover, but they will be infectious for two months.
Are there any symptoms other than the seizures?