Yeah, indeed, how horrible.
Seriously though, glad you got there in the nick of time. What are the odds ... and man I hate losing fish to jumping (or escaping as in the case of eels). What a sucky way to go. I can't believe I lost my canary wrasse when I had eggcrate on the tank .. did he have a one in a million shot and fit through the squares just right? Or did he have enough inertia to lift the eggcrate up, pass through over the glass, and the eggcrate just fall back down onto the tank? Guess I'll never know. (And yes I know I've complained about this a lot and yes it was a year ago but yes I really do obsess over things like this. I want to know so I can take steps to ensure it can't happen again!!)
I like your idea of a cover when unattended although that's something I see not working for me for very long. I prefer something permanent that I can circumvent just enough for ease of feeding, like a little flapper hole I can open up to get my hand through..