Cider Vinegar for chickens is acidic and it is this acidity that means ACV is a mild anticeptic and will kill a number of germs. In addition to this, it is claimed to act like a mild antibiotic. Antibiotics kill infectious bacteria that can cause disease in chickens. it has also been proven to help with stress.
You want the ACV that states on the label "With The Mothers." The health food section in Kroger carries ACV "With The Mothers." It is not pasteurized, it is raw. I put a few capfuls in a gallon every time I changed the water. It also helps a gal absorb calcium, if you have soft shelled eggs.
And will kill the smell of a skunk off a dog better than tomato juice!
Our health food store sells it, but it doesn't come cheep (sorry, cheap). But then I discovered our feed store (where I will be buying pellets, straw, etc) sell it in 5 litre containers for a lot less per litre than the health store. And since I'm an accountant, that saving makes me very happy.