Any fish that got flushed out will be lost, they need things to be 'just so'. I once helped clean up at a friends place who had 30 aquariums with thousands of fish after the power went out longer than his UPS could provide it. The mess was sickening. He never had another tank after that and it was his life's passion.
Interesting to think about. On the one hand, it's no different than any of the random stuff that happens to fish in the wild, or wildlife in general. Being killed for no reason at all is the default in nature; humans build societies and civilizations and develop technologies in large part to avoid it.
On the other hand, this feels especially sad for some reason.
We have an industrial unit which has a mice problem. Every time I go to B&Q (home hardware store for non-UK folk) with the intention of buying mice traps, I just can’t push the button. These mice are destroying our stock, but I just cannot do it. I would be a terrible fisherman.
Have you considered outsourcing? To a cat (or cats) or a pest control company. The pest control company will probably check and close off opportunities for mice to enter the unit, which cats won't do, so you might want to consider retaining both options.
Warehouse cats can be nice warehouse companions too (depending on what happens in the unit).
I've had to do some mouse removal work in my forest adjacent home, and it's not pleasant, but neither is having mice around.
We took our cat in, but she absolutely hated it. Then we thought she escaped for a few hours until we found her expertly hiding underneath some machinery.
In Costco they sell this cat food called ‘Maintenance cat’. Maybe we need one of those cats.
To be fair, you have to catch and release most fish in the UK which, whilst annoying for the fish, doesn't seem to do them much harm since there's e.g. a whole bunch of 30y+ old carp that keep getting caught.
You're allowed to keep a certain set of game and sea fish if they're big enough and from the appropriate place.
I'm sorry for your friend's loss and for the lost livestock, but it seem like it was simply a ticking time-bomb.
If your friend had 30 tanks and thousands of fish, they should have had a more involved backup plan that involved a dedicated generator in addition to a UPS. A small generator would have been a tiny fraction of the materials and livestock costs of running a setup like that.
It was the work of a lifetime. Power here has been so reliable that people forget that it can fail, and even though he was somewhat prepared it turned out not to be enough.
Agreed though that it was a ticking time bomb. As for better backup plans: even autostart generators for data centers can fail when you need them most. Backup plans end up with more and more layers until you think you've got it all and then some little oversight will get you. In his case he probably could have done more but that's only because we're looking at it after the failure, for decades it worked.
Most people in cities in the US have never experienced a power failure that lasts longer than an hour or so (and many places have NEVER had one). It's understandable that people don't think much about it.
Though the moment the UPSs hit 50% you might try to run to Home Depot and grab a generator, but by that time they'd all be sold out.