Woke up this morning to find another one dead, it's been months since the last died but this one showed some similar symptoms.
I noticed a few days ago it appeared to have symptoms of finrot, I added some salt to the water to help with this but it continued to worsen so I moved the fish to it's own tank where I added some treatment to the water. The finrot seemed to stop but the fish started having a cotton wool like substance growing on it and it's body became increasingly bent until it was unable to swim, then it died.
I'm worried there is still some kind of infectious disease affecting these fish. One possible reason for this fish falling victim is that it was the only one with telescope eyes and got less food than the others, so may have been weaker.