Tanks vary in how long they can last---mine will go 8 hours without distress. Overstocking means less time. If you can't get aeration in main tank and fish are showing distress, either go to a store that sells sporting goods and get a battery run bait bucket bubbler or stand on a chair and dip and pour pitchers of water from as much height as you can manage for one or two minutes every half hour or hour depending on size of tank and length of time of outage.