Cory hovering at the top - what to do?

[Musket]

Hi everyone.
in my 30 gallon long tank my 3 year old cory (silverish with black spots and stripes and a black dorsal fin) is suddenly sitting at the top of the water or middle of the tank. Just hovering.
I see part of his tail is missing. so I added pimafix and melafix.

Yesterday I added 3 panda corys (1/4" in size) after a 30% water change.
Water quality is great, right in line. Samples taken 8 hours after water change.

With this addition, there are now 5 panda corys. It was in the evening after adding the pandas that I noticed him swimming and hovering at the upper levels. My first thought was that the new pandas were all over his tank and actively searching for food. Perhaps they are stressing him out. Later, checking on him again, I saw the tail piece missing that is when I added pima and melafix.

I have never seen or heard of this behavior in a cory. Any suggestions at all?

 

[Musket]

Well, unfortunately when I checked on him a few minutes ago, he was on his side dead. No puffiness or red marks. No unusual behavior until last night. Whatever it was hit quick. Everyone else seems fine, but I'll be keeping a close eye on them.
Still would like any suggestions if anyone has ever heard of anything like this.
Thanks.

 

[Zagz]

So sorry for your loss. What other fish in the tank? Perhaps one took a nip at him? Are they in the same tank as the ram with the swollen eye?

 

[jdsunflower]

I am certainly not an expert, but I read somewhere that Melafix can damage the labyrinth organ (e.g., betta), and I thought catfish were labyrinth fish too....not sure about this, but might be worth checking out for the future?

 

[Musket]

Thanks,
no he was in another tank, in another room. No cross contamination or anything.
I will check into that Melafix and labyrinth fish. Thanks for the tip.