My power gets turned off for maintenance a lot, and with 5 fish tanks, it can be stressful. Thankfully, I picked very hardy fish. What I do is add 2 air pumps to my 90g, and because it is so large, it shouldn’t drop too much, and if it does, my Bichir can go down to mid 60’s. In other tanks, if it is days on end, I will rap it in blankets and put an air pump in. All fish of mine have survived power outages.