I had one die on me after being in perfect health and a fast grower. I can't really say what caused the swelling..but just a couple of days after seeing it like that it died. It could have been a blockage..or internal parasites.
I suspect that almost a year in a warm aquarium was not good for a cold water fish. I wouldn't doubt they need a few months of chilly waters. Not in stone..but so many cold water fish kept in aquariums just run through their lifespans.
It was my first time with them in an aquarium. My other try many years ago was outdoors in my pond at the time.