The medicated feed is just to prevent coccidiosis ONLY, not other diseases/illnesses. And if a chick already has cocci, the medicated feed will not cure them. You'll have to buy the proper medication from the farm store (but make sure it's actually coccidiosis). So buying medicated feed now probably isn't necessary.Also, when I got them they had been started on medicated chick feed, and I had to transition them to non-medicated chick starter, because that medicated stuff is EXPENSIVE. But I have another one that isn't a meatie and it is sick. So I am going to buy some medicated stuff for both cause they're mixed together.
Personally, I don't think it's necessary to use medicated after the first week or two. After that, I switch to normal. Once they have that bit of immunity to coccidiosis from the medicated feed, they should be good. And with my most recent batch of Cornish X, I decided to try the more natural approach and not do medicated. I just gradually introduced them to grass and dirt from the environment they would be moving out to. I found it worked well.
With the other chick that is sick, what are its symptoms?