I believe they were damaged by substrate (smooth gravel) rather than disease. Only one Cory is affected. I am considering emptying the tank and switching to sand substrate now, I thought the smooth gravel would be OK (it was there before the Cory's). Ntrates etc are all fine, I can think of no other cause, unless it was the dreftwood. They have only been in the tank for three weeks. All seem healthy and eating fine. If I switched to sand would that be OK for bristlenose catfish which also live there?
Has anyone kept Cory's with either Onyx sand or eco-complete? If so could you please give me your experiences with these? Do the Cory's do OK? I'm considering switching substrate to one or the other of these.