Old thread, but it's kind of been revived. It's cool you were able to rehab your acf, ALRtist, welcome to AC!
One thing people don't think about in situations like this is that animals are subject to some of the same physiological ailments as humans. I was reminded of this the day after Christmas when my Aussie-labrador mix had a stroke. We woke up to her blind, unable to move her rear legs and experiencing pain any time some one touched her below the chest. She was only 9 and was perfectly normal the day before. It was heartbreaking. Anyways, any animal that has a circulatory system can have a stroke, even fish and frogs, as well as heart attacks, seizures, blood clots, lung diseases in tetrapods, blood diseases such as anemia and leukemia, cancer, and so on and so on.
Then there are bad genetics, which are always going to be a possible issue in captive bred fish, especially in fish that are bred for certain traits, such as color, fin pattern, albinism, etc. Any fish could have a congenital disorder, but genetic problems are going to be higher in fish that have been bred for certain characteristics because obtaining those traits usually requires inbreeding to some degree, increasing the chance for harmful mutations.
As hobbyists we always want to know what killed or harmed our fish, but sometimes it just isn't possible to diagnose what happened and even if we could make a diagnosis we wouldn't be able to cure it. The only thing we can do is to make sure we provide the best conditions we can for our cold-blooded friends to reduce the chances of some of these things happening.
WYite