Please with urgency obtain durvet high level Vit B compound from your feedstore or Amazon. Dose 1ml orally daily but as your duck is struggling maybe give twice a day for a few days to ensure the full dose is ingested. You might soak 1ml into a few mealworms but it's difficult to ensure the duck gets the full dose. I just had two pekin ducklings that developed acute severe niacin deficiency with a growth spurt and at 5 weeks age I was able to add Durvet to their water. I made up half a gallon and added 8mls durvet. That way each cupful contained 1 ml. I used a 1 pint/2 cup Tupperware tub with 2 rounded off square holes in the top large enough for a duck bill. I stood the tub into a slightly larger heavy earthenware bowl (dog bowl) and gave my 2 the 2 cupsful of medicated water first thing in the morning when they were thirsty after not having access to water at night. I also gave them their duck pellets with 1 tablespoon of nutritional yeast flakes per cup of pellets sprinkled on top. They took their medicated water within 30 minutes each morning. In the afternoon when I was topping up their pellets and nutritional yeast, I gave them another 1 pint tubful of medicated water. Most of that was drunk before bedtime. At 5-6 weeks old, ducklings don't need food and water over night.
If you have to wait for Durvet to come from Amazon, you might buy human vit b compound. I found it at Walmart pharmacy but it's likely available at many pharmacies. Your duck will need 50mg niacin per day. If you are adding it to the ducks water, you will have to add 100mg for both ducks. You will not harm a healthy duckling giving it Vit B compound as Vit B is water soluble and excess is excreted through the kidneys as it cannot be stored in the body.
Ducks, particularly Pekins have high Vit B3 (niacin) needs that is not met by starter or all flock crumbles. Even duck crumbles may not have enough for a rapidly growing pekin duckling. I always add nutritional yeast to pekin duckling crumbles.
Good luck: let us know how your duckling gets on
