If you're having deaths it couldn't hurt to dose them with Corid just to be safe and see how they respond to it. Mine had all the symptoms of coccidiosis but tested negative in a fecal... the Vet and a BYC member advised me to medicate and I did, and I'm glad because they did start looking better after a few days. I don't know why it didn't show up in the nasty diarrhea samples I took in, or if the person who did the test didn't know what they were doing, but by the way they responded to the corid I don't think it was anything else.