I picked up a B Complex this am at walmart. For less than $3
The complex I'll be using has the following per tablet *supplemental information for feed weight in parenthesis from Merck Vet Manual
Thiamin 1.5mg (4 mg/kg)
Riboflavin 1.7mg (10 mg/kg)
Niacin 20mg (55–70 mg/kg)
B-6 2mg (3–4 mg/kg)
Folic Acid 400mcg (1mg/kg)
B-12 6mcg (20 mcg/g)
Adding this along with the niacin I've already been providing shouldn't be an issue, since I'm at 136mg/gallon of water, adding 1 pill to each gallon would bring me to 156mg/gal. That is, if I hadn't already upped the dosage by 50% last night. That puts each gallon at just over 200mg right now and 220mg/gal.
My plan is to supplement with the plain niacin @ 136mg/gal as I have been, plus the B Complex as the current supplementation clearly hasn't been sufficient. If the duckling exhibits other symptoms that suggest an neurological issue or a Vitamin E deficiency, I'll have to re-assess the situation and look for an alternate treatment course, but as I mentioned in a PM to @Amiga all the signs point to Niacin. I may also look into individual treatment with a syringe/feed tube if the duckling doesn't improve with flock wide treatment. This ensures that the duckling will be getting the treatment in the dosages I've planned and I'm not 'wasting' the treatment on ducks that don't seem to have a dietary issue.
Part of my reason for posting all of this here is that I feel the need to illustrate that sometimes things happen despite doing "all the right things." These ducklings have been provided the recommended feed and supplemental niacin in their water (which they don't swim in) plus a water soluble multivitamin, and STILL one of the 5 wasn't having their dietary needs met.