If you can believe that nonsense then you should also have no problem believing that perhaps they die because they hate their owner and would rather go to fish heaven instead. Hmm...let's see a 16in Azul, a 14in Azul, and a Lei....who's next?
Also, I believe he actually needed to jumped out of the tank in order to sacrifice his life for yours, according to superstition. Well, I guess you are still stick with your bad luck, some sort of impending death possibly (again according to superstition), after all.