Here's what the Merck Vet Manual says about amprolium (i.e. Corid):
"Amprolium is structurally similar to and is a competitive antagonist of thiamine (vitamin B1). Because rapidly dividing coccidia have a relatively high requirement for thiamine, amprolium has a safety margin of ~8:1 when used at the highest recommended level in feed. Maximal effect occurs about day 3 of the life cycle of coccidia. Because amprolium has poor activity against some Eimeria spp , its spectrum has been extended by using it in mixtures with the folic acid antagonists, ethopabate and sulfaquinoxaline"
I added the underline. What that means is that the effectiveness of amprolium has been extended with sulfa-based drugs because it is NOT effective against all types of the eimeria protozoa. The reason I suggested the Sulmet is because it has been proven effective against Eimeria Tenella, which is normally the culprit when bloody droppings are present. Not all strains of Eimeria attack the ceca or produce the symptom of bloody droppings. That's why coccidiosis is frequently misdiagnosed as something else - because it's generally believed that blood in the stool is always a symptom. It isn't.
Bottom line is - two popular products are used to treat coccidiosis - Corid (amprolium) and Sulmet (sulmethazine sodium). Corid is considered a coccidiostat meaning that it controls or diminishes the number of protozoa to non-harmful levels. Sulmet is a cocciocidal meaning that it kills all the protozoa. There are advantages and drawbacks to both types of treatment. If you wish to take a conservative approach, you could treat your birds with Corid. If symptoms do not subside or come back after treatment, you should turn to the Sulmet.
My personal preference is Sulmet, but then I don't hesitate to use antibiotics on my birds...but that's another argument for another day.