Have had many outages over the years where I live. Had a two day outage middle of winter some years ago. We had a propane heat to keep the main rooms in the house warm but the fish room got pretty cool and by the second day temps were down to high 60s to 70 in my tanks. Was breeding a lot of haps and peacocks at the time (and fronts). Didn't bother them. Kept O2 in the tanks with battery air pumps and I didn't feed for the couple of days. They slowed down their activity some until power and tank temps came back up, but no real issues.
Only fish at the time that was stressed was a wild rotkeil severum I had-- eyes got cloudy, but he recovered quickly and continued to live a healthy life-- since he was full adult size when I got him (or at least two years old) he'd have to have been at least 13 when he died.
Filters also recovered quickly, largely due to the cool temperatures, especially my fluidized bed filters (known for recovering quickly from power outages), but other filters back up to speed pretty quickly also. Had it been hot I'd have needed to do more to keep filters from turning sour.
