Corid is a brand name of liquid Amprolium 9.6%. It is usually labeled for pigs or cattle, but it's the same stuff you need for your chickens.
You medicated chick starter contains a small amount of Amprolium in it which is supposed to help them fight off infestation of cocci organisms, but it does not prevent. Actually, i think i am going to stop using it because with two flocks, i have used the medicated starter, and i have had to medicate for infestation both times.
The trick with coccidiosis is that it usually shows up in younger chickens and rarely in older chickens. However, delay of treatment (if it IS cocci) gives it longer to advance.
The good news is that once they have survived an infestation to a particular strain of cocci, they should be resistant to it for their lifetime - kind of like immunity.
Both of my flocks displayed signs of infestation almost exactly 2 weeks after gaining full, daily access to the out doors. These symptoms in older chickens who have been outdoors for a long period of time is more rare.
If i were you, i think that i would probably (i'm tentative to make this recommendation) wait another day and look for more bloody poop (or other symptoms) in the morning. If no more symptoms or bloody poop, and no lethargic chicks, i think i would just watch them and assume it was something else. If you do see anyone looking lethargic or more bloody poop - normally obvious on the roost - i would medicate with Corid immediately for 5 solid days in their water.
Also, in my opinion, it is a good idea to have Corid on hand, especially if you ever intend to have young chicks again. This way you can always medicate immediately when you notice symptoms.