Fish tank power outage: how to keep your fish alive without electricity

When the power goes out, the filter, heater, air pump and lights all stop at once, and the biggest danger is oxygen running out. The most important thing to do is get air into the water first (a battery air pump, or stirring the surface), then keep the temperature steady and don't feed.

Fish don't die from the dark. Without water moving at the surface, little new oxygen gets in, while fish, filter bacteria and rotting waste keep using it. Crowded tanks and warm tanks run out first, because warm water holds less oxygen (UF/IFAS). Meanwhile the temperature drifts toward room temperature, and the helpful bacteria in a filter with no flow start to suffocate within hours. Most short outages of a few hours are survivable for a healthy tank. Problems grow after about 4–6 hours, and sooner in crowded, warm or heavily fed tanks.

How to identify it

You'll know the power is out. What you need to watch is how the fish are coping: whether they gather at the surface, how fast they breathe, and what the thermometer says. Three things build up over hours:

Oxygen runs out, so fish crowd the surface and breathe faster.
Temperature drifts: cold in winter, hot in summer. The Merck Veterinary Manual lists power and thermostat failures as a cause of sudden deaths and of infections that follow when the immune system is weakened.
The filter goes bad. A sealed canister filter with no flow can turn stagnant and toxic, and may smell bad when the power returns.

For a summer outage, also see fish tank too hot. For a winter outage, see cold tank.

When it's a problem

Early: fish gathering near the surface and breathing faster; water temperature moving more than a couple of degrees from normal.
Serious: gasping at the surface ("piping"), with large fish affected first. A bad smell from a canister filter, or cloudy water when the power returns.
Emergency: fish lying still on the bottom or losing balance; fish gasping despite aeration; water reaching the mid-50s °F (about 13°C) in a cold outage.
Normal, don't worry: a few hours without power in a healthy, well-stocked tank. NC State's vet school notes many tropical fish tolerate the low 60s or high 50s °F (about 14–17°C) for several hours, and most fish can go days or even weeks without food.

What to do right now

Oxygen first. Put in a battery-powered air pump with an air stone or sponge filter. No pump? Stir or splash the surface with a clean cup or jug every 15–30 minutes, or pour water from a height back into the tank. Take the lid off, or prop it open, to let air in (keep jumpers in mind).
Turn off the auto-feeder and don't feed. Digestion uses oxygen and uneaten food fouls water (NC State CVM).
Keep the heat in (in winter). Close the lid partly and wrap the tank in blankets, towels, a sleeping bag, foam or newspaper. Large tanks hold heat well and cool slowly.
Keep the heat out (in summer). Shade the tank, open the lid, and fan the surface if you have a battery fan.
Unplug the filters and heaters, so that when power returns you control the restart.
Protect the filter bacteria if the outage will last more than a few hours: open a canister filter and move its media into the tank, or into a bucket of tank water with an air stone. Sponges from hang-on-back filters can go in the tank too.
Check the thermometer every hour or two, and write it down.

What to do next

Keeping air going for a long outage

Battery air pumps run for hours on D-cells or a built-in battery. To stretch them, Aquarium Co-op suggests running them in intervals once an outage passes 8 hours: for example, one hour on every eight, or every four hours for crowded tanks or if fish are gasping. Some pumps run from USB power banks or a car with an inverter. If you have a generator, run the air pump and heater first; the filter next.

Keeping warm in a cold outage

Once the water reaches the mid-50s °F (about 13°C), add heat.
Safe options: bottles of warm water floated in the tank; small additions of warm, dechlorinated water (about 10% at a time, every hour or two, checked with a thermometer); shipping heat packs taped to the outside glass. Aquarium Co-op suggests one heat pack per 75–110 litres (20–30 gallons).
Never pour boiling water into the tank. Mix hot water into a bucket of tank water and check it first.
A safe room heater can keep a fish room warm. Propane or kerosene heaters must be used only as their makers allow, with ventilation and a carbon monoxide alarm. Your safety comes first.

Keeping cool in a hot outage

Shade the tank, open the lid, fan the surface, and move small tanks somewhere cooler such as a basement. Avoid floating ice: melting causes sharp temperature swings. Oxygen is the bigger risk in hot water, so keep the surface moving.

When the power comes back

Check the filter before you restart it. If a canister or hang-on-back filter sat without flow for more than a few hours, rinse it (and its media) in tank water before plugging it in. Otherwise the first rush pushes a slug of stale, low-oxygen, ammonia-rich water into the tank.
Restart the heater and let the tank warm slowly, about 1°C (2°F) per hour.
Put the filter media back and restart the filter.
Test ammonia and nitrite within a few hours, and daily for a week. If either is above zero, do water changes (see ammonia spike). NC State suggests a 10–30% water change if ammonia passes 1 ppm.
Hold off feeding for about a day, then feed lightly.
Watch for ich and other infections over the next 2–3 weeks, especially after a cold outage (see cold tank).
Cloudy water after the restart is common (dead bacteria and waste). See cloudy water.

Cautions

Shrimp tolerate cool water but suffer quickly when oxygen drops.
Plecos and loaches need oxygen-rich water and often struggle first in a still tank. Labyrinth fish (bettas, gouramis) can gulp air and usually cope best.
Live plants are fine in the dark for a few days, but rotting plant matter adds to the load.
Don't mix fish from several tanks into one "safe" tank unless you must. NC State calls it a last resort because it spreads disease and causes fights.

Products that help

Battery-powered air pump with air stone or sponge filter: keeps oxygen going when the power is out. The single most useful item.
Spare batteries, or a USB power bank for pumps that can use one: stretch the pump through a long outage.
Sponge filter: run all the time in at least one tank so you always have live filter bacteria ready.
Thermometer: lets you track the temperature hour by hour.
Foam sheets or blankets: insulate the tank in a cold outage.
Shipping heat packs: taped to the outside glass for gentle warmth in winter.
Battery or USB fan: moves air across the surface in a hot outage.
Ammonia and nitrite test kit: checks the tank after the restart.
Generator, battery power station or UPS (for a fish room): runs the air pump and heater. Check that your pumps run on it.
Monitor with a phone alert: tells you the power or temperature failed while you're away.

Keep on hand before trouble starts: a battery air pump and air stone for every tank, spare batteries, and blankets or foam (plus heat packs in winter).

How to prevent it

Own a battery air pump and an air stone or sponge filter for every tank, with spare batteries.
Keep blankets or foam sheets handy for insulation, and heat packs in winter.
Run a sponge filter or air stone all the time in at least one tank.
Don't overstock. A lightly stocked tank lasts much longer without power.

Why it happened

Storms, grid failures, a tripped breaker or a pulled plug. You can't prevent outages, only prepare. Tanks that suffer most are crowded, heavily fed, warm, and have no backup air.

When to get help

If fish are gasping despite aeration, if ammonia keeps rising after power returns, or if you're losing fish in the days afterwards, ask a good local fish store (many will hold fish during a long outage) or a vet who sees fish. Honest limit: in a long winter outage with no heat source, small tropical tanks may cool faster than you can manage. Prioritise your rarest or most sensitive fish.

In the game

In Multiple Tank Syndrome, the City power failure card hits every player: "no filters, no heat, no air for a day". Every tank, for every player, is lost. The Battery air pump card saves all your tanks. That's the real lesson: a small battery pump is the cheapest way to survive an outage.