That’s a totally valid point about stress being a killer, in which case there might not be one culprit. Btw I highly doubt it’s the lion...he doesn’t bother anyone because there’s literally no fish small enough that he could swallow.
Wow just the dragon eels must have cost you like 3 grand at least. You should probably put a webcam on the tank then put something with a similar size and temperament as the victims. When they die you can review the footage and find out who is doing the killing. There are a lot of suspects on that list. Also don't rule out some crazy hitchhiker like a bobbit worm or something that could have gone unnoticed.