Here's the thing: IF it is coccidiosis, you don't have much time to act. Once they get really sick with it they die quickly. This may not be coccidiosis but if it is and you wait until you see even more decline it may be to late. Once chickens start actually showing that they don't feel well they are already pretty sick. They hide their illness very well and for as long as they can. If she has been eating her regular feed then I seriously doubt this is something that vitamins alone is going to solve.
It certainly won't do any harm to medicate with Corid. It's not an antibiotic, all it is is Amprolium which is a thiamin blocker. This creates an unfavorable environment for coccidia to overgrow. Once it get's to far along they sustain to much damage to the intestine and it becomes to late to treat.
My feed store carries the 20% soluble powder, dose for that is 1/2 teaspoon per gallon of water for 5 to 7 days. I'm not sure what the dose is for the 9% liquid.