Sick chick- what to do?

[ErinJones0810]

I have 4 Barred Plymouth Rock chicks. They just turned 3 weeks old. Last week, I noticed one of the chicks having symptoms of coccidiosis (was weak, not eating, and diarrhea). I treated all the birds with Corid for 5 days. After the first day of medication, all the birds appeared more energic, they were eating and happy. Today (which is the first day off the Corid), 3 of the chicks look great, very happy and active. But one chick is lethargic, head drooping, not eating or drinking, and had stringy clear-whiteish diarrhea. Nothing that I do seems to make her more alert. I separated the one sick chick from the others so I can closely monitor. What should I do?