You probably wouldn’t need much to have a back up sorted, and once a month trip the switch to ensure it works properly. It is as easy as a relay, breaker (switch, for testing), battery, and a secondary pump. You can keep the battery topped up by a trickle charger. You could also get something to message or call you when the back up is activated. We also have that for the alarm system. It has 3-4 phone numbers it calls to alert someone of a problem. You need a flow alarm and backup power supply for a small circulation pump. My neighbor lost him and got a pump and supply that turns on when the flow meter you put inline stops and circulates at half power of an AC pump it also has a way to wire it to a light bulb to show it’s on. It came from a catalog. I want today McCoy pond products.
I grew a koi for two years in a 5 ft round 2ft deep pond in the summer and a 75gallon indoor tank in the winter twice that big. How the hell did he keep them so small? Also had three softball-sized goldfish in there. Those things wouldn’t die. Seems like something major went wrong. They shouldn’t need power for days in a worst-case scenario. I’m so sorry about your uncle. Jokes aside, I also have a koi pond and every fish has a name. We get babies for most years and I love them all. This would be a real loss to my husband and I. Hope he is able to start again.