Aquariums During Power Outages

How to keep fish alive when the power fails: maintaining oxygen, retaining heat, using battery air pumps, and protecting the biofilter.

Priorities

When the power fails, the order of concern is oxygen first, temperature second, and biological filtration third. Most fish can wait through a short outage, so there is no need to act in the first hour or two; the focus is on preventing oxygen depletion and large temperature swings over longer outages.

Maintaining oxygen

Without aeration, dissolved oxygen can fall within a few hours in a warm, stocked tank. A battery-powered or USB air pump restores surface agitation and gas exchange and can run for many hours, and longer on a power bank or deep-cycle battery. One practical approach is to run aeration for about an hour every 8 hours, more often for heavily stocked tanks. Manual stirring or scooping and pouring water also adds oxygen if no pump is available.

Retaining heat

With the heater off, insulate the tank: keep a tight-fitting lid and wrap the tank in blankets, towels, or emergency Mylar sheets. Heat packs can be used at roughly one per 20-30 gallons. If adding warmth, mix warmed water in a bucket first rather than pouring hot water directly into the tank. In hot weather, move the tank out of sunlight and add small amounts of cooling.

Protecting the biofilter

Beneficial nitrifying bacteria need oxygen and can begin to die when flow stops, with some sources noting decline within several hours of stagnation. To protect them, remove media from canister or hang-on-back filters and place it in the tank where it stays wet and oxygenated, and unplug the filters so stagnant water and decaying organics are not flushed back into the tank when power returns.

Do not feed

Stop feeding during the outage and for about 24 hours after power returns. Uneaten food and extra waste would raise ammonia at a time when filtration and oxygen are already compromised. Fish tolerate short fasts well.

After power returns

Clean filters before restarting them, then bring equipment back online. Test the water for ammonia and nitrite over the following days, as the biofilter may need time to recover its full capacity.