I've messed up I believe, I'm new to chickens and I brought in 4 older chickens from craigslist to my flock of 25 3 week old. After a couple weeks a couple of the older chick's started with crusty eyes and always scratching, and slinging snot around, and making this litl cough sound. Now a couple of my little ones r scratching at their eyes and shaking their heads. What could it be, should I remove the older birds, is it to late, and can my chick's b saved? Also they are all on medicated chicken starter and vitamin water.