Thanks for the speedy replies and helpful links.
I was hoping it would be old age (of all the things to hope for!). I think it's more likely to be that then anything else right now. I see no signs of illness or disease in him or any of the other fish.
The others are still picking on him a bit...
For the last few weeks I've noticed that one of my danios has become very slim, is almost transparent, has developed a bent spine and it looks like one of it's gills might be damaged. It's still attached about looks to be 'pushed' in. I'm assuming that it may have gotten into a lil scrap with...