Hard to come up with lifespans in captivity. In some way we may extend the life of a fish beyond what they might live on the reef through predation. Sick fish dont live long on the reef. As you point out Paul, fish can get cancer, loose eyesight and all manner of things we can't treat. A copperband that can't judge where something is might be easy pray on the reef not so in our tank. I guess your copperband will waste away unfortunately. Natural lifespan is IMO a shot in the dark. If somebody keeps a fish 10 years did it outlive it's normal lifespan or die early? Few fish on the reef will live till.old age I would guess but it's just a guess like guessing the natural mage of a fish when we have little in the way of real data.