04 Aquarium
Power cuts and aquarium emergencies
In a power cut the thing people worry about is the cold, and the thing that can do the damage is oxygen — for the fish first and for the filter bacteria straight after. Goldfish tolerate low oxygen better than many aquarium fish, but at room temperature that means hours, which is time to do the right things.
The power goes off, and the first thought is that the tank will get cold. It is the wrong thing to worry about.
What actually stops is the movement of water, and with it two supplies of oxygen: the one keeping the fish alive, and the one keeping the filter’s bacterial colony alive. The second is the one that can do lasting damage, and the easy mistake when the power returns is to switch the filter straight back on.
What stops, and what it means
The filter. Two losses at once. The mechanical part stops, which matters little for a few hours. The biological part stops being supplied with oxygenated water, which is the part to watch — the bacteria described in the cycling guide are aerobic, and in a sealed canister they are sitting in a finite volume of water they will exhaust.
The air pump. The tank’s surface is now the only gas exchange there is.
The heater. The slowest problem, and the one everyone thinks of first.
The light. Live plants make oxygen only while they have light, so in a planted tank let daylight in if you can.
The clock
A rough timeline for an ordinary goldfish tank at room temperature, from keepers’ experience rather than measurement:
| Elapsed | Fish | Filter |
|---|---|---|
| Up to a few hours | Usually fine; a crowded or warm tank can run short, so agitate the surface and watch for gasping | Restart it when the power returns |
| More than a few hours | Keep agitating the surface; keep the tank unfed and insulated at the back and sides | Sponge or internal filter: restart it. Canister or hang-on-back: open it, rinse the media in tank water, then restart. Test ammonia and nitrite daily for a week |
Two things shift that table. Stocking — a crowded tank runs out of oxygen sooner. Temperature — a warm tank holds less oxygen while the fish use more, so a summer power cut is a different event from a winter one.
First: oxygen
Everything here is about getting gas exchange happening at the surface, which is where it all occurs anyway — the aeration guide explains why surface area rather than bubbles is the mechanism.
A battery air pump. The right answer, and it costs about the same as a bag of fish food. Check the rated run time on the box and keep spare batteries. Some models sit permanently plugged into the mains and switch over automatically when it fails, which means you do not have to be at home or awake.
Manual agitation. With no pump, fill a jug from the tank and pour it back from a height as often as you can, more often in a warm or crowded tank, and watch the fish for gasping. It is an old keepers’ trick, not a measured routine.
Leave the surface open. More surface exposed to the room means more gas exchange, so insulate the back and sides but do not seal the top. The caveat is jumping, so a loose cover is the compromise for a tank you are leaving overnight.
Stop feeding. Immediately, and see below.
How long was it off?
The canister decision depends entirely on the duration, and if you were out or asleep you do not know it. Several things in an ordinary house do:
- A digital clock that has reset — oven, microwave, alarm clock — tells you it happened but not how long. A clock that resets to a flashing 00:00 and has been counting since is a timer of the outage’s end, which with the current time gives you the duration since power returned.
- The router, which usually reports its uptime in its admin page. That is the most precise answer most homes have.
- A smart meter or energy monitor, which logs the gap directly.
- A cheap plug-in power-failure indicator, sold for exactly this, which flashes and records that a cut occurred.
If you genuinely cannot tell, assume it was long. Opening and rinsing a canister that did not need it costs twenty minutes; not opening one that did risks sending fouled water into the tank.
The cut you slept through
The realistic version of this problem, and worth planning for because the decision has already been made for you.
You wake up, the power has been back for two hours, and the canister has been running the whole time — pumping whatever was inside it into the tank. If that water had fouled there is no undoing it, so the response is the same as for any sudden ammonia event:
- A large water change immediately, 50 percent or more, conditioned and temperature-matched, as the water change guide describes.
- Increase aeration and leave it increased.
- Do not feed for at least a day or two.
- Test ammonia and nitrite, and repeat the change on the readings.
- Then open and rinse the filter anyway, because whatever is still in there is still coming out.
- Watch the fish for 48 hours. Gasping, clamped fins or red streaking mean ammonia rather than the outage itself.
This scenario is also the whole argument for a mains air pump that switches to battery automatically. It does not need you to be awake.
Check the filter is actually running
A small thing that catches people, and it is worth knowing before rather than after.
Not every filter restarts itself. A canister that has lost its prime may hum without moving water, and one that has partly drained can run dry. An air-driven sponge restarts immediately. A hang-on-back usually restarts, but not always — if its impeller chamber drained, it will run without lifting water.
So when the power returns, look at the outflow, do not listen for the motor. Water actually moving is the only confirmation that counts, and a filter humming quietly with no flow for a day is a filter that is not filtering.
The same check applies to an air pump with a stiff diaphragm, and to any pump behind a check valve that has siphoned back.
A sump needs one extra check before the power comes back: where the water actually went. The drain-down happens in the first minute of an outage, so look at the sump level before restarting the pump — and at the biological media. What the cut did to it depends on the design: media above the water has sat in air, so the risk is drying out, while media still submerged in a stalled sump is the oxygen-poor case, as in a canister. Sumps and large systems covers designing that drain-down so it cannot overflow.
Second: do not feed
Stop as soon as the power goes, and do not start again until the filter has been running normally for a day.
The reasoning is direct: food produces ammonia, both as waste and as anything uneaten, and with no biological filtration there is nothing processing it. A tank with a stalled filter and a fish being fed is accumulating ammonia with no way out.
A healthy, well-conditioned adult goldfish can generally go a few days without food, and up to about a week in a cool room, as the fasting page sets out. This is one of the easier decisions available.
Third: heat, and slowly
A large volume of water is a large thermal mass and loses heat gradually. In a heated house that has only just gone cold, a tank may drop two or three degrees overnight, which for a goldfish is unremarkable — they are temperate fish and the temperature guide covers how wide their tolerance actually is.
What they do not handle is a fast change. So the job is to slow the loss rather than to replace the heat:
Insulate the back and sides. Blankets, towels or polystyrene sheets, but leave the surface open or loosely covered so gas exchange carries on. It costs nothing.
Close the curtains and shut the door of the room.
Do not add hot water to the tank. Pouring in a kettle produces exactly the fast local change you were avoiding, and a thermal gradient the fish have to swim through.
If you must add heat, do it outside the tank: a sealed bottle of warm water floated in it, replaced as it cools, raises the temperature gently and controllably. Never a naked flame near a glass tank.
When the power returns: the filter
This is the step worth knowing before it happens, because it is easy to get wrong when the power comes back.
Flow supplies the media with oxygen. Nitrifiers tolerate oxygen-free spells better than most bacteria, and no one has measured how fast a stopped filter’s bacteria die, but water sealed inside a canister or external filter can turn oxygen-free and foul within hours.
Switch it on and that water goes into the tank at once.
So, after more than a few hours off:
- Do not plug it in. Disconnect and open the canister.
- Pour the water away. All of it.
- Rinse the media in tank water, not under the tap: chlorine and chloramine can damage the biofilm that is left, and there is nothing to gain by risking it. A swish in a bucket of tank water is enough.
- Reassemble, prime and start it.
- Test ammonia and nitrite daily for a week, and treat the tank as partly cycled until they stay at zero.
A sponge or internal filter can simply be restarted, because it has been sitting in the tank’s own water throughout, sharing whatever oxygen the tank had. It is one of the quiet arguments for having one, alongside its role in a hospital tank. The filter media guide covers what is in each type.
A hang-on-back filter is treated like a canister here: if it has been off for more than a few hours, tip the chamber out, rinse the media in tank water and restart it.
When the power returns: everything else
Check the heater. Watch it against a thermometer for the first day: heaters can fail stuck on, and a stuck heater is a faster emergency than a cold tank. If the water level dropped and the heater was exposed, do not restart it until it has been submerged and equalised — thermal shock cracks the glass.
Bring the temperature back slowly. If the tank has dropped several degrees, let the heater do it over hours. Do not set it high to catch up.
Test ammonia and nitrite daily for a week. If the filter lost any capacity it has to rebuild, and the pattern will look like a small version of a cycle. The testing guide covers the readings and what to do about them.
Feed lightly for several days, and build back as the readings stay at zero.
Water changes on the readings, not on the calendar, for the first week.
Watch the fish. The healthy baseline page is the reference; clamped fins, gasping or lethargy in the days after a cut usually means ammonia rather than lasting damage from the outage itself.
Ponds
Mostly a non-event. A pond has a large surface area exchanging gas continuously with the air, a large volume that changes temperature slowly, and — in winter — fish whose oxygen demand is very low.
Three exceptions:
A heavily stocked pond in hot weather, where the pump and any waterfall were doing real aeration work. Manual agitation or a battery pump covers it.
A pond where a pump keeps a hole open in the ice. If the power is out during a freeze, melt a hole with a pan of hot water rather than letting the surface seal. The pond guide covers why a sealed surface is the actual winter hazard.
A pond filter that has been off for more than a few hours, which can have the same problem as a canister on a larger scale. Rinse the media in pond water before restarting.
What to have before it happens
A short list, and the first item does most of the work:
- A battery air pump, an air stone and airline, and spare batteries. Kept somewhere findable in the dark.
- A thermometer that does not need power.
- Blankets or towels, which you already own.
- A large jug, which you also already own.
- A test kit that is in date, for afterwards.
- A torch, for the same reason.
Optional and worth it for a large or valuable tank: a small uninterruptible power supply. Check its rated run time against what you plug into it, since an air pump draws less than a filter.
Other emergencies, briefly
A filter that has failed rather than lost power. The same rules apply: after more than a few hours off, open it and rinse the media in tank water before restarting.
A heater stuck on. Unplug it immediately, and cool the tank slowly with small water changes rather than ice. The heaters page covers the hardware and the failure mode.
A leak. Covered on the placement page, where the realistic cause is a filter hose rather than the tank itself.
Something in the room air. Aerosols, paint, cleaning sprays, cooking sprays and smoke all enter the water through the surface and through the air pump, and this is a genuine and under-recognised way to lose a whole tank at once — one of the answers on the why did my goldfish die page. If anything has been sprayed in the room, turn off the air pump, open a window, cover the tank, and do a large water change afterwards.
A prolonged outage of days. The fish should cope if the oxygen measures above are kept up. The filter may need re-establishing, so plan for it: light feeding, daily testing, and patience. If the house itself becomes uninhabitable — no heat in deep winter — the fish travel better than the tank does, and the moving guide covers how.
The short version
Oxygen first: battery pump, or a jug and a bit of height. Stop feeding. Insulate rather than heat. And when the power returns after more than a few hours, open the canister filter and rinse the media before you plug it in.
Frequently asked questions
How long can a fish tank go without power?
For the fish, usually several hours at room temperature: goldfish tolerate low oxygen better than many aquarium fish, but a warm or crowded tank has less in hand, and occasional manual agitation of the surface helps. The filter is the weak point. Flow supplies the media with oxygen, and nitrifiers tolerate oxygen-free spells better than most bacteria, but no one has measured how fast a stopped filter's bacteria die, and water trapped in a sealed canister can turn oxygen-free and foul within hours.
Can I just switch the filter back on when the power returns?
Not a canister or hang-on-back that has been off for more than a few hours. Nitrifiers tolerate oxygen-free spells better than most bacteria, and no one has measured how fast a stopped filter's bacteria die, but the water sealed inside can turn oxygen-free and foul within hours, and switching the filter on sends it straight into the tank. Open it, pour the water away, rinse the media in tank water, reassemble and then start it. A sponge or internal filter can simply be restarted, because it has been sitting in the tank's own water the whole time. Then test ammonia and nitrite daily for a week.
What is the single best thing to own for this?
A battery-operated air pump, with a stone and a set of spare batteries, kept somewhere you can find it in the dark. They cost very little, they solve the actual problem — oxygen — rather than the one people worry about, and the rated run time on the box tells you how long a set of batteries lasts. Some models plug into the mains and switch to battery automatically when the power fails, which removes the need to notice at all.
Should I worry about the tank getting cold?
Less than you would think, especially with goldfish. A large body of water loses heat slowly, and goldfish are temperate fish that live perfectly well at temperatures well below a heated tank. What matters is the rate rather than the number: a drop of a few degrees over a day is unremarkable, while a fast drop is a stress. So insulate the back and sides — blankets, towels — to slow the loss, leave the surface open or loosely covered so gas exchange continues, and do not try to add heat with hot water.
Should I feed the fish during a power cut?
No. Stop as soon as the power goes and do not resume until the filter has been running normally for at least a day. Food is where the ammonia comes from, and with no biological filtration there is nothing processing it. A healthy, well-conditioned adult goldfish can generally go a few days without food, and up to about a week in a cool room, which makes this one of the easier decisions in fishkeeping.
What do I do about a pond in a power cut?
Usually nothing. A pond is a large volume with a large surface area exchanging gas with the air, and in winter the fish are barely consuming oxygen anyway. The exceptions are a heavily stocked pond in hot weather, where a pump was doing real aeration work, and any pond where a pump is the only thing keeping a hole open in ice. In both cases, manual agitation or a battery pump covers the gap.
What should I check when the power comes back?
The filter first — if it is a canister or hang-on-back that was off for more than a few hours, opened and rinsed rather than simply switched on. Then the heater, which is worth watching against a thermometer for a day. Then temperature, brought back gradually rather than in one step. Then test ammonia and nitrite daily for a week, in case the filter lost capacity, and feed lightly meanwhile.
Why do goldfish cope with this better than many other fish?
Because they cope with low oxygen unusually well. Goldfish and their close relatives can survive extended periods of low or absent oxygen, switching to a metabolic pathway that produces ethanol and excreting it across the gills rather than accumulating lactic acid. It is a genuine physiological peculiarity, and it works best in the cold: wild crucian carp survive months of oxygen-free water under ice, but at room temperature the margin is hours, and a warm, crowded tank with a bloom can still run short. It is not a licence to leave them in it, but it does mean you have time to act.