I just read some reviews for the Dumor starter and it looks like someone else had problems with their ducklings as well, it sounds like it could be a niacin deficiency, which can cause neurological problems (like seizures, which seems like what your ducks are having) I would get some plain (not no-flush or timed-release, just plain niacin) niacin capsules or brewers yeast, which you can find at natural food stores. it's recommended to give them 100-150 mg of niacin per gallon of water, if using brewers yeast I believe it's about a tablespoon per duck on their feed everyday, I'm not positive, though as I use the capsules.
I have a mallard that was having seizures but hers wasn't a deficiency, I think it was some weird disorder, that she grew out of. But since you are having multiple ducks doing it, I'd say it's more likely to be a deficiency. I hope your ducklings better soon, it's very disturbing to watch them having these "episodes"!
I think you are confusing niacin B3 with thiamine, B1.