Extended power cut: how to save your aquarium?
During a power cut, an aquarium holds out 2 to 6 hours without intervention; beyond that, two emergencies in order: oxygen first, temperature second. With three accessories under 30 € prepared in advance — a battery-powered air pump, a blanket, a thermometer — even a 24-hour outage can be crossed without losses.
What happens in the tank, hour by hour?
As soon as the filter stops, circulation ceases and dissolved oxygen drops — the faster the tank is stocked and warm. After 2 to 4 hours, the bacteria in the filter media, deprived of flowing water, begin to die and release toxins. The temperature, meanwhile, loses only 1 to 2 °C per hour in a heated room: it’s almost always oxygen that kills first, not the cold.
What should you do immediately?
- Start a battery-powered air pump (10 to 20 €, the fishkeeper’s best insurance) or, failing that, stir the surface: a jug of tank water poured from a height, every 30 minutes.
- Take the media out of the sealed filter after 2 hours and rest them in the tank: the bacteria breathe there instead of rotting.
- Insulate the tank: a blanket or polystyrene sheets over the panes and the hood.
- Do not feed: digestion consumes oxygen and polluting without filtration makes everything worse.
- Open the hood only to stir the water: heat escapes from the top.
How do you manage an outage longer than 12 hours?
Keep stirring regularly and watch the temperature: below 18 °C for a tropical tank, add sealed floating bottles of hot water (heated on gas or at a powered neighbour’s) — never hot water poured straight in. A planted, lightly stocked tank, low-tech style, holds out noticeably longer: the plants produce oxygen as long as there is daylight. Put the tank in indirect natural light if possible.
What should you do when the power returns?
Restart the filter and heater, check that the pump has properly reprimed, put the media back, then test ammonia and nitrites for three days: if the biological filtration has suffered, a mini-spike is possible — respond with 20 to 30 % water changes. Then restock your emergency kit, detailed in our safety and prevention category, and note how long the tank held: that’s your real margin for next time.
Frequently asked questions
Is a UPS for the aquarium worth it?
For a large tank or an outage-prone area, yes: 60 to 120 € to power the filter and an air pump for several hours. The heater, too power-hungry, stays with the blanket.
Are coldwater fish more outage-resistant?
Doubly so: no heater to compensate for, and cold water dissolves more oxygen. A tank of White Cloud Mountain minnows sails through a 24-hour outage almost without noticing.
How long do the filter bacteria survive?
About 2 hours in a sealed filter without circulation, several days if the media soak in the oxygenated tank: hence the importance of getting them out quickly.
This guide is part of Planète Pets’s Fishkeeping universe. Our advice is general in nature: for any health concern, your veterinarian remains the only reference.