Sudden death in half-grown chickens?

[squidchicken]

We have incubated/raised our own eggs for about 3 years now. We are on our third generation of chicks and they are about 3 months old (or about half grown). We've never before had an issue with illnesses or anything like that but all of a sudden in the past four days, we've lost 4 chickens (1 per day). The first one was pecked by the big chickens to death, so we separated the big and little chickens, as we combined them too early. But then others started dying and we cannot tell what's wrong. They look normal, but they act really weak. They have a hard time standing/walking, but they do drink and eat a little. We went to our local feed store and bought Duramycin-10 Tetracycline Soluble Powder and put that in their water and then we bought medicated chick starter/grower. We also got a treat/nutrient block just for good measure. Is there anything else we can do to stop this? There is one chicken we caught before it died and it currently has white feces around it's vent and is acting really weak.
They are like pets to us and we really care about them. Any answers are appreciated, and thank you for reading and helping!!!!

 

[Eggcessive]

What age chickens are dying? In multiple aged chickens, coccidiosis can be a problem in younger birds before they have built up resistance to cocci in the soil. Heat in summer can also be a problem. Make usre chickens are all drinking water well, and keep water in shade with multiple water stations available. If you lose another, contact your state vet for a necropsy, but be sure and refrigerate the body.

 

[squidchicken]

The about 3 month old ones are dying. They have an outside area and an inside area in their coop. The water is inside in the shade and they seem to be drinking until they get sick. And we'll check into that. Thank you!

 

[EMorical]

Progress report? Have a similar situation going on here as well!