I have had a couple of 'cory disasters' recently and this has upset things in my tank quite a lot. Corys are one of my favorite fish, and back before I joined this forum I kept them very successfully for some years, especially remarkable since I didn't really know what I was doing!
However, more recently whenever I've tried to keep corys, it's been a complete catastrophe. I bring home seemingly healthy fish (of course, you can never tell if they might be harbouring some invisible disease) and within a month they're either dead or slowly dying. Meanwhile, all the other fish in the tank with them (previously healthy) become ill themselves just from the stress of having sick fish in the tank. It's incredibly depressing!
A few months ago I bought some very healthy looking Peppered corys but these died mysteriously, just like the last shoal of corys I had. I had 4 Cherry Barb and 5 glowlight tetras in my 15 gallon tank (which has now been established for 2 years.) While there were sick Corys in the tank, one of my glowlight tetras got fin rot that completely destroyed his dorsal fin (it's now growing back) and one of my cherry barbs also got finrot.
When my last cory died, I thought the sick tetra and barb had recovered, but a few days ago I noticed the barb looking very unwell indeed. There were no external signs of disease like fungus or finrot (that I could see) but the fish was shivering all over and having trouble swimming. She seemed to really have to struggle to keep herself afloat.
The other fish in the tank were all healthy and the last thing I wanted was to have a sick fish releasing stress hormones into the water so the first thing I did was isolate her in a separate bowl. I still can't really see anything wrong with her - the edges of her fins are slightly ragged and dark looking and the tip of her pectoral fin is bloodshot but that doesn't seem to explain why she's so sick. She really is too weak to move now - just sits there on the bottom breathing very slowly.
I've added a few drops of Myxazin to the water but I honestly think it's too late to help her.
I didn't want to post this in the 'disease' forum because I really can't afford to buy $100 medications at the moment. But I feel bad about not being able to do anything...