It's possible they have coccidiosis which may require medication. Twelve weeks is about the time it starts to manifest if it's going to. You could take a stool sample to a vet, it usually doesn't cost all that much to test it. It'll need to be a fresh as possible.
One possible thing you could try is cayenne pepper in their food. The chickens don't taste spicy so it doesn't bother them. I've never done it but the recommendation is to put enough in their food to turn it reddish-pink. Or try some fresh parsely; it's supposedly got some antiparisitic qualities.
Another option is to try some food grade Diatomaceous Earth, feeding that to them. It's a little iffy because if they're at the point where they're having loose stools and it is cocci it could get bad pretty fast.
If their stools turn bloody and they start looking poorly you'll need to make a decision to medicate or you could lose your flock. It's a bummer when you're trying to be as organic as possible but you might think of it in terms of our own medication. If I need to, I'll take medication to alleviate an illness despite my mostly organic diet.
Other than these suggestions I don't know. You could go up and do a search for cocci or loose stools and see what comes up.
Good luck, Mary
