Alright, I posted about this a while ago, but I thought I'd post again to say sonic has survived!
He got in contact with a female who had a disease. I think it was dropsy, as she pineconed and died. A while later, sonic became inactive. I put him in quarintine. Then he pineconed and bloated, and I thought he would die. But he held on for two weeks of no eating. Then he recovered, very skinny, but alive. Then, he gained wait and is now happily living in his divided tank. Does anyone know what his disease was?