Someone suggested taking Night's body for a necropsy. If that's not possible and you have another die, take it right away for a necropsy to find out what you are dealing with. So sorry to hear about your troubles. I've been through that and it just sucks!
I agree with the others who suggested a necropsy. I would want to be able to find out exactly what it is. Otherwise, you're probably just guessing unless one of your other chickens displays some verifiable symptoms, you know?
With this many people not being able to give you any decent answers......it just seems like a necropsy makes the most sense....to me.
We decided to send Night in for a necropsy. I hope it isn't horrible enough that the flock cannot be treated... the vet said to continue sulmet, so we will. Once again, thank you everyone!
After you are finished with the ten days or two weeks or whatever the Sulmet treatment is, you should consider giving the chickens some plain yogurt to reestablish good bacteria.
You may want to buy some PolyViSol baby vitamins without iron and give each bird a drop on the beak three times a day for a few days.