First of all I agree with what others have said, and I'm sorry for your loss. I don't think all of your chickens could have botulism. You might have something like coccidiosis happening, or a sub-clinical case of mycoplasma. I would give them a round of Corid for 5-7 days just in case. There are a lot of viruses out there that can affect poultry, so gettin a necropsy done would help you figure things out.
