Did you see my posts?Come to think of it, the Silkie laid a tiny, unusual egg. And the plymouth's egg was soft shell. I was suspecting parasites. I thought it might've been fleas, as my cat had health problems, which all vanished when we killed the fleas.
How do you know whether to treat for cocci or parasites? Or do you just do both?
More info would help.
How old are birds?
What and how exactly are you feeding?
Have you checked them over real well for mites and/or lice?
Best done well after dark with a strong flashlight/headlight, easier to 'catch' bird and also to check for the mites that live in structure and only come out at night to feed off roosting birds.
Wipe a white paper towel along the underside of roost to look for red smears(smashed well fed mites).
Part the feathers right down to the skin around vent, head/neck and under wings.
Google images of lice/mites and their eggs before the inspection so you'll know what you're looking for.