If they are running around they probably don't have it. (eta: Coccidiosis)
I was just saying....
16 in a few square feet (it can't be more then 6 or so sq ft huh?)
is TOO small.
Do you have a coop big enough built for them yet?
Feeding medicated feed does not necessarily prevent coccidiosis nor will it cure it if it does get out of hand.
What size if this playpen?
Month old chicks need LOTS of room.
Can you post a picture of your set up?