You are right, and I would treat too.Yeah, it's a killer. I would definitely recommend treating as quickly as possible. I think Corid is actually amprolium. Which is what is in the medicated feed. Corid is at a higher concentrate though. There are other things you can use, but I think Corid is the least damaging to their system while it takes care of the coccidia.
Understand that this is what I've gathered from my research. Please, someone with more knowledge, correct me if I'm wrong.
Coccidiosis is nothing to mess around with if you suspect it's a possibility. I would use Corid, my avian vet strongly recommended against the sulfa meds, they are very harsh on a birds system. Coccidiosis gets all out of control and it can go really fast. By the time a chick is so sick that it can't help but show signs of illness it may well be to late to treat. Corid is such a simple med that to me, if I see signs, it's well worth it to treat for 5 days rather then risk loosing a bunch of chicks.
Corid is simply Amprolium. Amprolium is, pretty simply, just a thiamin blocker. It is not an antibiotic. It won't get rid of the cocci altogether, it simply creates an environment that is unfavorable for it to overgrow. Therefore the name coccidiostat. There will still be some cocci in the gut, that is how the chick is then able to develop some immunity.
Corid comes in liquid or powder. I've never used the liquid just because powder is what my feed store sells so I don't know the dose for that. The dose for the 20% soluble powder is 1/2 teaspoon per gallon of water for 5 to 7 days.