Moldy feed causes aspergillosis, a fungal infection. I don't think antibiotics treat fungus. http://www.thepoultrysite.com/diseaseinfo/7/aspergillosis/
Corid is an anticoccidant. Coccidia is a parasitic disease caused by a protozoa. These protozoa are all over the ground and thrive in wet conditions. Chickens can eventually build resistance to them. But they can be overcome with them quickly before they have this resistance. It causes diarrhea, sometimes with blood in it, sometimes not.