Yea, it does seem that way, doesn't it? All the ones I was most attached to passed before their time. Seems like we try to do everything we are suppose to and they still fall ill.
On the other hand, I return home for summer vacation and the trio of way-too-big-for-their-tank-now koi, feeder goldfish, and sailfin pleco in my dad's tank are still alive and still thriving (*knock on wood*), my dad having given up on fixing and filling up the pond he got them for ages ago. That tank gets one 100 percent W/C every handful of months—more when I try to take care of it—and it has been this way for the last six years.