I agree medicated feed is not really neccessary. I keep Corid on hand if I see a couple really runny poop in a row from a couple chickens/chicks when they are young and use it as stated above. I know they say they cant get it till they are out in the run but I am pretty sure mine came to me with it from the feed store one year. One of mine almost died and the rest were showing signs within a week. I treated with corrid and all was eventually fine. Earlier you asked about grit, I let them have a few days to make sure that everyone is eating well and then I put a grit container in the brooder, it gets them ready for treats that I can never resist giving them after the first week. You do have to keep an eye and make sure they are all getting plenty of actual feed but I haven't had a problem ever.
What you may not know is that now, coccidiosis has mutated. There is a form of it that can actually be passed down through the egg to the chick from the parents-whodathunkit?! I know this only because of a necropsy done by the University of KY on a friend's chicks.
This was a few years ago. A breeder sent me hatching eggs to hatch for a friend who was coming down for a visit, a surprise gift she wanted to give as she was also a friend of my friend. The chicks were in my brooder for less than two weeks when she took them home,
along with chicks of my own that I hatched. A couple of months later, they began to die, just seemed lethargic one day, dead the next, one by one. No symptoms. She did treat for cocci, but it didn't work. It was so mysterious that she had one tested and it was a mutated form of cocci that was not known in that region and that was
not one of the 9 normal known types; they say it passed through the egg and the chicks already had it inside them. That was a shocker for both of us. None of my chicks in that same brooder with them ever had anything wrong with them, so it was specific to those olive egger chicks. The vet said you must give them a
double dose of Corid, not the regular one, but double strength, that a regular dosage would not touch that type of cocci they had.
Germs and other organisms are mutating. It's not that strange that the minimal dose of amprolium in the medicated feed does not work well anymore. That's one reason why I say medicated feed is practically useless now.