I have a 29-gallon established tank.
2 juvenile angels
4 black skirt tetras
1 female betta
1 male swordtail
4 zebra danios
2 baby cory cats (used to be 3 adults)
2 red-eyed tetras (that will hopefully be finding new homes soon)
GH: 120ppm (as best as I can tell)
KH: 220ppm (as best as I can tell)
Water changes: 15% twice a week
I had three adult cories for several months; all was going well until about 5 weeks ago one of them started listing off to the side, gasping for breath. He obviously was not going to make it, so I euthenized him before he expired and got eaten.
A week later, I could not find a second cory, and never found another trace of him.
Three weeks went by and my remaining cory was happy and healthy. I bought two more baby cories, and all went well for two weeks.
Yesterday, I found my remaining adult cory upside-down and thought he was dead. I took him out and put him in a separate tank where he twirled over and over, gasping for breath just as the first cory had. He died soon after removing him from the big tank. Since this one was an albino, I was able to see that his gills were much too red to be normal, and that one side was more pronounced than the other. I cannot get my photo to attach.
No other fish have taken ill, only the cories. Is this some kind of parasite, like twirling disease? Could it be gill flukes? Are my other fish in danger? Thanks for any suggestions. I know the info given may not be enough for a diagnosis, but just thought I would ask.