Pawnmower is right. You just need some surface agitation to promote gas exchange. Otherwise the fish will start to get oxygen-deprived. If a power outage goes on for a long time (like for a day or more) that could be a problem as biological filtration will start to break down. The bacteria that process nitrogen are largely aerobic and if they lack oxygen, they’ll start to die. That can lead to ammonia build up or worst case, having to re-cycle the tank all over again. The starvation of aerobic nitrifying bacteria would be even worse with a sealed canister filter so it might be worth using a UPS if you have a canister and the power is off for a long time. Run it for a few minutes and then unplug it to save battery. I guess temperature drop might be a concern during the winter but again, we are talking extended outages. Only issue there is that I’ve seen issues with tank heaters damaging UPS batteries and ruining their effective life span.
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