Most feed stores have it in the cattle medicines. Tylan Soluble Powder or Tylan 50 injectable are used in chickens. The powder is used in the water for 5 days, the injectable is given by a shot into the breast muscle for 3-5 days, or it may be given orally as well for 5 days. Amoxicillin can...
The undigested seeds and foul smelling droppings may be clostridium perfringens and enteritis. Enteritis can be treated best with amoxicillin or Tylan. A fecal test could tell you if there is a ton of cocci (all chickens will have some) and a vet could help if it is enteritis. Tylan Powder...
Here are some good links to read about coccidiosis, but if you lose another chicken to whatever disease is happening, I would get a necropsy done by your state vet:
http://www.merckmanuals.com/vet/poultry/coccidiosis/overview_of_coccidiosis_in_poultry.html...
Yes, it is more common in chickens under the age of 20 weeks, but there are two strains (out of the 9 or more affecting chickens) that cause a more chronic form. Also, past coccidiosis can lead to enteritis in older chickens. Treating with Corid is pretty harmless, but if the OP gets a stool...
Diarrhea and puffing up, not eating,loss of one chicken--I would start Corid (amprollium) right away in their water. The liquid Corid is easier to give a drop or two undiluted before mixing it in the water, and any chickens not drinking well need to be given the Corid with a dropper. Dosage is...