I have had a bunch of rasboras and one lonely black neon tetra for almost 8 years now. All of a sudden, I have had a mass exodus of fish in the last probably 3 months or so. The tetra isn't looking good right now and i am down to one last rasbora. I have been kind of expecting them to die of old age sooner or later, because I think 8 years is a pretty good life span for this type of fish, but I was thinking about it today, and got to wondering, how do you know if fish are dying of old age or if they are dying because of something else?