Like not in his tank or gone as in died? I'm so sorry though
I know it probably doesn't help but more often than not we try to pin deaths on medications or diseases or anything but sometimes they can just die. It's the same thing in humans, sometimes we just drop dead and even the most advanced scientist doesn't know what happens. And in that, we tend to not like it, we as humans have to point a finger somewhere and we end up pointing it at ourselves and it's not our fault no matter how you look at it. I just want you to remember that, I'm sure you know and it probably still won't help the more I blabber on about human tendencies, but I just wanted to say it in hopes.
Swim in peace little Atlas.