The treated water needs to be the only water they have access to, or they won't get enough of the medication. So, yes you may need to lock them up to accomplish that. Are your hens showing any symptoms? Coccidiosis is very common, and older birds usually have some resistance to any strains they have been exposed to, and won't get sick unless exposed to a new strain or have weakened immune systems. How soon after you brought in the new birds did they get sick, and how old were they? The question is whether they brought it with them, or picked up a strain on your property. It's everywhere, very common, as I said. Not uncommon for birds to get sick with it when moved to new ground as they can be exposed to new strains there. Corid is very safe, so if in doubt, treat. If it's on your property, it's impossible to get rid of it really. Wild birds can bring it in, you can carry it in on shoes and clothing. Don't make yourself crazy trying to get rid of it. Keep droppings from building up, keep feeders and waterers clean and free of droppings, try to keep the area as dry as you can, standing water and wet mucky conditions can lead to break outs.