This ^ is an important point. Fish in nature die, generally speaking, from natural causes. Mankind causes the death of many, to be sure, but most simply are eaten by predators. Very few fish, or any animals, live long happy lives in nature and then die of old age; those that do are very lucky.
Fish in aquariums might die of old age, but again, those are rare. Most will die due to our silly mistakes, neglect, bad luck, ignorance, or other "unnatural" causes. A tetra that is eaten by a pike cichlid in nature has died a natural death; when the same thing happens in one of our tanks, it's our fault, because we put those two fish together in the first place. Same thing with disease; in nature, a certain amount of disease-induced death is inevitable...but in a fish tank the diseases usually hit as a result of poor water conditions or lack of quarantine or some other circumstance the we have unnaturally created.
In fact, I'd wager that very few fish actually die in nature due to disease; rather, they become slower, weaker, less able to escape predation...and thus are killed and eaten before the disease runs its course. It's only in our tanks that diseased fish are "lucky" enough to get the opportunity to suffer through the entire prolonged painful disease process until it kills them.
I owned a large Jelly Cat once upon a time for about a decade; I was one of a short string of owners that fish had during a captive lifespan that stretched out over at least 30 years...and even that fish died during a botched transfer attempt from one owner to another, rather than succumbing to the almost-mythical "old age".
Yup!
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