When the power goes out, your fish are not in immediate danger. Most aquariums are fine for a few hours. The two real risks are falling oxygen, because the filter and air pump stop moving the water, and falling temperature, which matters most in winter and in small tanks. So during an outage: insulate the tank, stop feeding, add oxygen by stirring or pouring water, and keep an eye on the temperature. If the outage lasts more than a few hours, clean the filter before you restart it. Below: what happens hour by hour, 7 steps to follow, and the cheap kit that makes the next outage a non-event.
Contents
- How long can fish survive without power?
- What to do during a power outage: 7 steps
- What should I do when the power comes back?
- How do I prepare my aquarium for the next outage?
- Frequently asked questions
How long can fish survive without power?
There is no single number, because it depends on the tank size, how many fish you keep and how cold the room gets. As a rule of thumb:
| Outage length | What happens | What to do |
|---|---|---|
| Up to 2 hours | Almost nothing. Temperature and oxygen barely change in a normally stocked tank. | Do not feed, keep the lid closed, wait. |
| 2 to 6 hours | Oxygen starts to drop, especially in crowded tanks. Small tanks in cold rooms lose several degrees. | Insulate, add oxygen manually every hour, watch the fish. |
| 6 to 24 hours | Bacteria in a closed canister filter run out of oxygen. Temperature can fall to room level. | Keep aerating, add warmth, clean the filter before restarting. |
| More than 24 hours | Real risk for tropical fish in an unheated room. | Battery aeration, warm water bottles, consider moving fish to a warm place. |
Big tanks are safer than small ones. A 200 litre tank holds a lot of heat and loses it slowly; a 20 litre nano can drop to room temperature in a few hours. Lightly stocked planted tanks are safer than crowded tanks, because they have more oxygen per fish.
What to do during a power outage: 7 steps
Step 1: Stop feeding
Digestion uses oxygen and uneaten food pollutes the water while the filter is not running. Healthy fish can go several days without food, so skipping meals during an outage costs nothing.
Step 2: Insulate the tank
Close the lid and wrap the tank in blankets, towels or sheets of polystyrene, leaving a small gap at the top for air exchange. Insulation is the single most effective way to slow down heat loss in winter.
Step 3: Add oxygen by hand
Without a filter or air pump, the surface goes still and oxygen exchange slows down. Once an hour, scoop a jug of water from the tank and pour it back from a height, or stir the surface gently for a minute. The aim is surface movement, not a whirlpool.
Step 4: Use a battery or USB air pump if you have one
A small battery-powered air pump, or a USB air pump connected to a power bank, keeps oxygen up for hours with no effort. It is the best investment for anyone who lives where winter storms cut the power. Our Schego pumps are mains powered, but they can run from a UPS (uninterruptible power supply) for a long time because they draw only a few watts.
Quiet German-made air pump for nano and shrimp tanks. Low energy use, built to run for years.
Step 5: Keep the temperature up
Check the temperature every hour or two. If it drops below about 22°C for tropical fish, fill plastic bottles with warm (not hot) tap water, close them tightly and float them in the tank. Replace them as they cool. Never pour hot water straight into the aquarium, and change the temperature slowly: a gradual drop of a few degrees is far less stressful than a sudden swing back up.
Step 6: Leave the lights off
Darkness keeps fish calm and lowers their oxygen use. In planted tanks, plants also stop photosynthesising in the dark and start using oxygen, so do not shine strong torches into the tank for long periods.
Step 7: Watch for warning signs
Fish gasping at the surface, crowding near the top or breathing rapidly mean oxygen is running low. Increase manual aeration straight away. Fish lying still on the bottom in a cold tank are usually just slowed down by the temperature, not dying.
What should I do when the power comes back?
- Short outage (under about 4 hours): simply switch everything back on.
- Longer outage with a canister filter: many aquarists recommend opening the canister and rinsing the media in tank water before restarting. Water that sat still in a closed filter for many hours can turn foul, and you do not want to pump it straight into the tank. Hang-on-back and sponge filters are less affected.
- Check the heater: make sure it is fully submerged and working, and let the tank warm up gradually.
- Test the water the next day: check ammonia and nitrite. A partial water change of 20 to 30% (temperature matched) helps if anything is off. See our guide on why fish die after sudden changes.
- Check timers and dosing pumps: clocks may have reset. Chihiros dosing pumps with a CR2032 backup battery keep their schedule and catch up on missed doses.
- Feed lightly the first day.
How do I prepare my aquarium for the next outage?
A few inexpensive items turn a power cut from an emergency into a minor inconvenience:
- A check valve on every airline. If the air pump sits below the tank, water can siphon back down the tube when the power goes off and flood the pump or the floor. A check valve costs a few euros and stops this completely.
- A battery or USB air pump with a charged power bank, kept with your aquarium kit.
- A battery-powered thermometer that reads without mains power.
- A UPS for the air pump (and the filter, if it is small). Air pumps use very little power, so even a small UPS keeps them running for a long time.
- A correctly sized heater, so the tank recovers quickly afterwards. Our heater size guide shows how many watts your tank needs.
- Insulation ready to use: a polystyrene sheet under and behind the tank keeps heat in all winter, not just during outages.
Stops water siphoning back into your air pump when the power goes off. Fits standard 4/6 mm tubing.
Mounts outside the glass and shows water and room temperature, so you see how fast the tank is cooling.
Be ready before the next storm
A check valve, a reliable air pump and a thermometer that works without power. Small kit, big peace of mind.
Shop the essentials →Frequently asked questions
How long can fish live without a filter?
In a normally stocked tank, fish are usually fine for several hours without a filter, and often a day or more with manual aeration. Crowded tanks and large fish run out of oxygen faster.
How long can an aquarium go without a heater?
It depends on the room. In a heated home, most tanks lose only a few degrees overnight. In a cold room in winter, a small tank can reach room temperature within hours. Most tropical fish tolerate a slow drop to around 20°C for a short time.
Should I feed my fish during a power outage?
No. Skip feeding until the power and filter are back. Food increases oxygen demand and pollutes the water.
Will my filter bacteria die during a power outage?
Bacteria on sponges and media in open filters survive a long time. In a closed canister filter without flow, oxygen runs out within hours, so for longer outages rinse the media in tank water before restarting.
Can I use a power bank to run an aquarium air pump?
Only with a USB air pump designed for it, or through a small inverter or UPS for a mains pump. Air pumps use very little power, which makes them the best device to keep running in an outage.
Want to understand what the numbers mean after an outage? Read our water parameter guide for beginners.