There are only a couple strains of cocci that cause blood in feces in chickens, the other 7 strains do not cause blood in feces.The symptoms you described does sound like Cocci. The bloody poop is one of the last signs/symptoms to appear. Just to be safe, I would go ahead and get some Corid and mix it in with the water. It will not hurt any chicks if they don't have Cocci. Lots of people ( myself included ) give the Corid water as a preventative. It basically just slows the growth of the Cocci so the chick has a chance to build an immunity to it
