If you haven't found a vet who sees waterfowl, it would be very worthwhile to find one. Usually the hatchery will be able to recommend a practicioner. At some point in these ducks' lives they're likely to need antibiotics or other treatments that will require diagnosis and prescription.
From what you're describing I'm not so sure this is a nutritional effect. It sounds more like an infection, which could be transmissable to the other. It's always best to head these things off at the very first symptom rather than be dealing with some long standing complications.
When it comes to the nutritional needs, pelleted formulas have become a good option even with waterfowl, but the pleasure of variety is still necessary for a happy duck.
Pelleted diets are highly recommended and preferred because of a far lower risk of molds and bacteria growth which often happens in other food sources like mash and grains. Studies are also showing without doubt that pelleted ducks grow bigger and quicker than others.
For supplemental foods offer some hard cooked eggs, regular garden snails with or without shells, worms/night crawlers, bloodworms and of course cracked corn. It's important to be sure when offering slugs and the like that you need to be certain no pesticides or other chemicals were used in the area and could possibly in the flesh.
For a printable guide
http://www.duckhealth.com/ducknutr.html
Too much protein can be nearly as much of a problem as too little. A condition called ‘angel wings' occurs when overfed proteins. The feathers grow up and out rather than sleek. When too little protein is offered, walking/flying and balance problems, along with more serious health problems will result.
Sometimes adjusting the diet will correct problems caused by insufficient nutrition; however, it's not always possible to reverse the effects. What is most important is that caregivers at least try.
Again though, I'm not that sure about what you're seeing. I have to strongly recommend establishing a vet.
Please let me know how you all make out and I wish you all the best!
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Animal Rescue&Care Org Owner; Animal Care author; Animal Behavior Consults.;Cert. Avian Specialist;
Have you tried calling the local ASPCA or Humane Society? They will often have a list of qualified vets that they use for rescues. Also, local fish and game have to have someone.
It sounds like you're being very responsible and caring - losing the little ones you've lost already had to be hard. I'd like to see these two really thrive and grow to be very, very old ducks - seeing your children through their high school years!
http://www.pwrc.usgs.gov/BBL/homepage/long1290.htm