Yes, coccidiosis kills fast. All birds are exposed to it from the soil,but sometimes their bodies get overloaded and they can't fight it. It's not a disease that you need to cull for - they can fully recover from it. Older birds rarely get it because they have been exposed to it and have developed an immunity to it.
Corid is amprolium. It is not an antibiotic. It is an amino acid that blocks thiamine. The Protozoa that causes coccidiosis is sensitive to it, so it helps the birds recover and develop an immunity to it.
Here is the dosage - 1/2 tsp per gallon. Don't mix anything else in the water with it.
https://www.backyardchickens.com/t/548021/need-help-with-corid-dosage#post_7025207