Our chicken Honey has (had) a recurring yeast infection in her crop because her crop was damaged because of starvation as a young bird so it's distended and liquid pools in it. This is a breeding ground for yeast infection. Although treated with very expensive Fluconazole (aka Diflucan), once the yeast infection went away, our vet recommended we give her treated water to keep the yeast infection from recurring. The recipe:
1 teaspoon of unfiltered organic apple cider vinegar per 1 quart (32 ounces) of water.
Our vet said this should be weak enough that she'll still drink the water without noticing something is in it, but be strong enough to keep infection at bay. She said this works with other animals, too.