Forgot to mention, there are many products you can buy online without a veterinary prescription required, they usually go into the chickens water for a week. In the UK, harkers coxoid is the one used, not sure about the US. I've seen mixed reviews on harkers coxoid, some say it worked, others say it didnt and their birds didnt make it. The veterinary prescribed medication I was given was baycox, I treated my birds with it for two days, the blood in poop stopped a
It's likely coccidiosis, you'll need a veterinary prescribed medication to kill the coccidia and a disinfectant designed specifically for coccidiosis to clean the whole area with to stop re infection. Everyone has a different opinion when it comes to coccidiosis, some believe building a natural immunity is the way to go, and small amounts of coccidiosis are nothing to worry about, a vet will always advise medication, even if they find only one coccidia in a faecal sample, as it can be life threatening to the chickens, I know this from personal experience with my vet vs suggestions from chicken keepers on forums like this one! I went with the medication option, by all means do your own research on it and make your own decision! I'd say by the time you're seeing consistent blood in their poop, a medical intervention is probably required...
I can take pictures of the poop if you need them.