I personally do not use medicated feed (cannot purchase that here without a vet anymore!) or water though I have a gallon (smallest size available to purchase from a vet) of Amprolium on hand now, I have very rarely had to use it. It’s actually the overload, or sudden exposure to, and not the organism itself that is the killer. If your brooder was clean (and it sounds like these were your first chicks and it should have been fairly uncontaminated, although I haven’t really been keeping up here enough) there shouldn’t have been enough contaminated fecal matter to overload an otherwise healthy chick.
My first chicks I hatched out from eggs and had in a pristinely clean brooder until they were several weeks old, I then purchased some week old chicks from the SAME breeder I got my hatching eggs from. They had been brooded on her farm and exposed to her chickens and the coccidia there, but they were overall healthy and active. I set them up in a brooder somewhat near my older chicks. Cross contamination occurred and my big healthy older chicks got suddenly very ill. The very first sign of sickness was a dead pullet, I believe they were 6 weeks or a little older. The younger chicks that seem to have brought the Coccidia into the environment never suffered more than the occasional pinkish poop from intestinal shed, no truly bloody poo, puffiness, lethargy etc.
I had a very hard time actually getting the Amprolium but with treatment in their water, moving them outside, and daily moving onto new ground, all my other chickens came around and are mostly doing well to this day.
In very young chicks it is quite hard to see the symptoms I find, as they are tiny, fluffy, and don’t have exactly normal postures to begin with. There is also the tendency to eat, drink, eat, flop down and look dead but actually be sleeping and perfectly healthy in very tiny chicks, you don’t see the passed out pancake chick when it’s under mama, but under a heat light or at the edge of a heat plate it’s pretty distressing. Is it sick? Or just sleeping?
Now I am also running meat chickens (in addition to the layers) so I am regularly bringing bulk batches of hatchery stock 40-60 birds at a time, into my overall flock and putting them in the same brooder and running them over the same ground as my layers. I do occasionally lose a chick to coccidiosis, but it will be just one, maybe two out of that many birds. It is always a runt or less healthy looking chick that seems to become ill from it and I believe it is related to another underlying health concern in the individual. When I notice symptoms I directly treat the individual, but often by the time the symptoms manifest the chick is too ill to recover.
I’ve switched my focus from completely sanitizing the brooder environment, to just maintaining general cleanliness, so chicks can adapt and build a resistance to the local coccidia, and so far I haven’t had as many issues. I still clean my incubators with a mind to sanitizing them totally, but the brooders I just focus on temperature, humidity, and keeping the poop levels down, easier in a smaller batch, but with 60 growing chicks it piles up fast, add in the warm environment, our natural humidity and all those little breathing babies... it’s a potential breeding ground for all sorts of nasty bacteria. So lots of ventilation and frequent little changes become imperative