I might have missed it so just in case: It’s worth mentioning that coccidiosis is very common when a chick (or any baby animal) has already been weakened by something else— like a respiratory infection. And it can then take hold in the other chicks. From the stool shown, I’d start Corid immediately, dosed as suggested earlier.
It wasn’t said in the previous post, but the Corid water needs to be made new daily, and should be the only water offered (can be given in multiple waterers & “recipe“ can be halved—1 teaspoon in 1/2 gallon H2O— to avoid waste).
Perhaps, hopefully, it wasn’t really coccidia & your chick is now fully recovered after the antibiotics had more time to work