No natural selection is what occurs in the wild. A death sentence is whats going on here.
While I do believe you when you say the eel was going for the goldfish, i have no doubt that the lions time was coming either way. i've seen green morays take down huge lions w/ no fear or consequence of the venom. A LFS had a dragon moray take down two very large black tips, while he couldn't eat eithers shark, he sufficently mamed them that they died. Tessy are beautiful eels that belong by themselves, just like green morays. They don't fear larger fish, and hence think everything in striking distance is food.