I'm not advocating for overstocking, but, a battery backup would probably have saved me. The thought crossed my mind occasionally. It was just a major misfortune that the power happened to go out while I was out of town. When it rains it pours I guess. The good news is my Midas was totally unaffected since he's alone in a 75 as well as my Flowerhorn.
Hello; I follow your thinking. I have never tried a battery backup so do not have any practical experience. I guess the battery would last a while if running only an air pump. Might not run power filters, heaters and lights for long. Perhaps a separate air pump that is plumed into an air line but is not on until the line power is out? I Do not know how to set this up to be automatic when away.
I guess there are at least three sorts of stocking levels. There is what I now do in which I try to keep the level at the point a tank can run for days without any power inputs. There is stocking levels that are fine with the power filters and bubblers running and the tanks may run OK for a few hours during an outage. The stocking levels that are ,in my opinion, too dense are those where even with power filters and such running a tank is on the edge of problems at all times.
I know there are related issues to consider. Overfeeding and tall tanks come to mind.
Anyway, I have had losses over the 50+ years and can understand how you feel. I came home from a three day weekend to find one of my larger tanks had opened a seam and leaked out. Had floor damage as a bonus to the tank loss as the tank was a display in the living room. No more carpet around tanks and I cover the floors in my tank room with commercial vinyl tile. I also have a drain hole into the basement to give the water a way out.
Good luck in the future. Good that some fish made it.