Put them on the full dose treatment of Corid the day before you put them out, that gives the medication time to get in them. The medication mimics thiamine, which the coccidia need to survive, they feed on the medication instead and starve and die. You may have a high load of coccidia in your environment or a particularly virulent strain, so the Corid will help prevent the numbers from multiplying in your chicks guts and overwhelming them. Do the full 5 to 7 days of treatment, make sure it's the only water they have access to. Medicated feed contains a very small amount of the medication, but it does not always prevent outbreaks, when it happens you have to use the full strength treatment dosing.
Correct dosing is 2 tsp of the liquid Corid per gallon of water, or 1 1/2 tsp of the powdered Corid per gallon of water, make fresh daily.
When on medicated feed, it only works as a preventative if the chicks are exposed to the coccidia while on it. Once they are exposed, and recover, they will have some resistance to that strain in the future. If they are never exposed then they cannot build resistance. My brooder raised chicks get a saucer of dirt from my yard (not from the chicken run) starting day one that they can dig and scratch and peck and dustbathe in. They are naturally exposed to all the microbes and have a chance to build resistance while their immune systems are developing. I always have Corid on hand in case treatment is needed, but by doing this I have not had to treat in many years, I don't have chicks getting sick when they finally go outside on the ground. Corid is pretty safe. If doing it this way does not make any difference then I would send one off for necropsy when they die, find out for sure what the cause is.
