Power cut aquarium kit: air first, heat second

What actually kills first

Most power-cut advice starts with blankets and heaters. That is the wrong end of the problem. When the electricity goes, your filter stops, the surface goes still, and the tank stops exchanging gas with the room. Meanwhile the fish keep breathing, the bacteria in your filter media keep breathing, and at night so do your plants. Consumption carries on; resupply stops.

Water holds a surprisingly small amount of oxygen to begin with, and warm water holds less. Fully saturated fresh water carries roughly 9 mg per litre at 20 °C, about 8 mg at 25 °C, and about 7.5 mg at 30 °C. A warm, heavily stocked tropical tank is therefore starting from the worst possible position: the highest demand and the lowest ceiling.

Temperature works on a completely different clock. A 200 L tank in a 15 °C room loses roughly one degree every two to three hours, and slower still with a lid and a blanket over it. A 20 L nano loses heat much faster, but a 20 L nano also has far fewer fish breathing in it. In almost every household outage, the oxygen problem arrives first and the cold problem arrives later - which is exactly the opposite of the order most people act in.

Build the kit before you need it

The three things that actually matter

• A battery or USB air pump. These draw very little power. A typical USB nano pump uses 1-2 W, so a 10,000 mAh power bank - roughly 30 usable watt-hours after conversion losses - will run one for somewhere between fifteen and thirty hours. A two-D-cell battery pump gets similar mileage from a set of alkalines. Either way, you can keep a tank breathing overnight for the price of a takeaway.
• An air stone and airline. The stone is not doing what people think. The bubbles themselves transfer relatively little oxygen; the value is the surface disturbance they create as they break, which is where the actual gas exchange happens. A cheap stone plus a metre of airline is enough.
• A check valve. Fit it in the airline, above the water line, the right way round. When a pump below the tank's water level stops, water siphons back down the airline - through the pump, onto the floor, and out of the tank. A 50p part prevents a genuinely expensive night.

The rest of the bag

• Spare batteries or a charged power bank - kept charged, not kept meaning to charge.
• A battery headlamp. Hands free matters when you are working on a tank in the dark.
• A separate thermometer that does not need power to read.
• Dechlorinator - for the water change you may need afterwards, not during.
• A liquid test kit for ammonia and nitrite. Test strips are fine for routine checks; after an outage you want the sharper reading.
• Blankets, towels or a sheet of foam board to insulate the tank, plus whatever lid you own.
• A clean jug that has never seen soap, for manual agitation.

Honest limits on the bigger options

A small UPS, the kind sold for computers, will run an air pump for many hours because an air pump draws almost nothing. It will not meaningfully run a heater. A 200 W heater will flatten a typical desktop UPS in well under an hour, and you will have spent the battery on the problem that was not going to kill anything first. If you own a UPS, put the air pump on it.

Petrol generators are a real answer for long rural outages, but never run one indoors, in a garage, or near an open window - carbon monoxide from a generator kills people every winter. A car inverter running an air pump from the engine is a safer improvisation, with the same ventilation caveat.

The first ten minutes, the first four hours, overnight

First ten minutes. Get air moving - battery pump on, or start manual agitation. Put the lid on and leave the lights off. Switch the CO2 off if you inject it (more on that below). Note the time; you will want it later. Do not open the canister, do not feed, do not panic-change water.

First four hours. Insulate. Lid on, a folded blanket or duvet over and around the tank, curtains closed. Leave a small gap for the airline. Check the thermometer every hour and write the numbers down - the rate of fall tells you whether heat is going to become a real problem or not. Keep the tank dark; fish are calmer and use less oxygen.

Overnight and beyond. This is where a planted tank gets harder rather than easier: in darkness plants consume oxygen instead of producing it, so a heavily planted tank's overnight oxygen demand is higher than its daytime demand. Keep the air going. If the temperature has dropped several degrees, accept it and keep it stable rather than trying to correct it without power. A steady 20 °C is much kinder than a sawtooth between 18 and 24.

Heat without power: what is realistic

Honestly, not much, and that is fine. Tropical fish tolerate a slow drift down to 20-21 °C considerably better than they tolerate suffocation or a thermal whiplash. Your realistic tools are insulation and thermal mass, and they work better than people expect: a lidded, blanketed 200 L tank in a 14 °C room will often hold above 22 °C for the best part of a day.

Nano tanks are the exception, because they have almost no thermal mass. A 20 L desk tank in a cold room can shed five degrees in a few hours. Floated bottles of warm - not hot - water, changed periodically, are a reasonable stopgap there. Wrapping matters more the smaller the tank is.

If the outage is long and the room is genuinely cold, the sizing conversation is the one in our winter heater checklist and the heater size guide: the fix is a properly sized heater and an insulated tank before the next storm, not heroics during this one.

When the power comes back

1. Check the heater is fully submerged before it energises. If evaporation or a spill has left it partly exposed, unplug it at the wall first. This is when heaters crack.
2. Restart the filter carefully. Air-driven sponge filters, internal filters and hang-on-backs are straightforward - switch them on. A canister that has stood still for more than a couple of hours is different: the media inside goes anoxic, and simply switching it on pushes stagnant, sometimes sulphurous water into the tank in one go. After a long outage, disconnect it, tip the old water away, rinse the media in tank water or dechlorinated water, reassemble and then restart.
3. Let the temperature come back slowly. Do not reach for a bigger heater to catch up. Around 0.5 °C per hour is the pace.
4. Test ammonia and nitrite before you feed anything, then daily for about a week. Nitrifying bacteria are aerobic, and a long outage will usually knock the cycle back to some degree. If ammonia shows, treat it as new tank syndrome and manage it with small water changes and light feeding.
5. Feed lightly, late. A day without food does a healthy fish no harm at all. A tank full of uneaten pellets after an outage does real harm.

Why overstocked tanks fail first

Every fish you add shortens your outage buffer. The oxygen budget is fixed by the water volume and the temperature; the demand is set by how much fish you put in it. Two tanks of the same size, one at 60% of a sensible stocking level and one crammed to 110%, do not experience the same power cut - the second one is in trouble in a fraction of the time.

This is one of the practical arguments for stocking conservatively that rarely gets made. If you are not sure where your tank sits, run it through the free stocking calculator - it will tell you your bioload as a percentage and flag the limiting factor. The inch-per-gallon rule will not, which is one of several reasons to stop using it.

The planted and CO2 caveat

If you inject pressurised CO2 through a solenoid, the power cut turns your CO2 off for you. If you run a needle valve with no solenoid, or a DIY yeast reactor, injection carries on with no circulation and no surface exchange - dissolving CO2 into a tank that is already losing oxygen, and dropping the pH while it does. Turn it off manually, first thing.

Running an air stone in a CO2 tank during an outage is exactly the right call. You will off-gas the CO2, and that is the point. Plants will be fine for a day. Fish will not.

If fish are already gasping

Gasping at the surface during or after a power cut is an oxygen emergency, not a sign of a sick fish. Increase agitation immediately - pump, stone, jug, whatever you have - and lower the temperature no further than it has already gone. The differential and the next steps are in fish gasping at the surface.

If fish have already died, or are dying after the power returned, do not guess. Work through Tank SOS, which will take you down the outage and oxygen branches in order, and read suddenly losing fish for the wider picture. The most common post-outage killer is not the outage itself, it is an ammonia spike from a knocked-back filter two days later.