mysterious ailment

[clutterydrawer]

Hello
I don’t know what section this belongs in, it’s not an emergency as such, just something I’d like to know. Hopefully someone can help me out.

A few times now, each several weeks apart, I’ve lost a fish to a disease which causes internal inflammation, weak, disorganised swimming, and very quick death. There is a pic of the end result here and if anyone could offer any ideas to what’s causing it I’d really appreciate it because I'm worried that the whole tank might go this way.

Thanks for any suggestions.