You can take the liquid internally, but you can over do it. Put 1 teaspoon of it in your duck's water. I think if you did that once a day it would be good.
Silver shield is an awsome infection killer. Kinda expensive, but I've used it on numerous chickens before and it works amazing. You can either get a gel or liquid.
https://www.naturessunshine.com/product/silver-shield-rescue-gel-24-ppm/?msclkid=fe6e325c3acd1d4b4ca55613dac95683...