Heater or filter failure: the emergency kit that saves the aquarium
A heater or filter failure doesn’t forgive improvisation: a 50-80 € emergency kit, prepared in advance, is the difference between an incident and a wipe-out. The two emergencies run on different clocks: a stopped filter leaves you a few hours; a heater stuck on heat can cook a tank overnight.
What goes in the emergency kit?
- Battery-powered air pump (10 to 15 €) with an air stone: keeps the oxygen going through a filter or power failure.
- Spare 50-100 W heater (15 to 25 €): a new budget unit in reserve beats a shop that is closed on Sunday.
- Independent thermometer (3 to 8 €): never trust the heater’s own light alone — see our guide to the heater and thermostat.
- External thermostat plug (15 to 25 €): cuts the heater if it runs away — the deadliest failure of all.
- Clean water bottles and a blanket: to warm or insulate the tank during a winter power cut.
- Water tests and conditioner, already on hand if you follow our maintenance checklist.
Filter down: what to do, hour by hour?
First hour: check the plug, an impeller jammed by a snail, a blocked intake — 80 % of failures end there. If the filter is dead: plug in the air pump, stop feeding (less pollution, less oxygen demand) and take the filter media out to keep it submerged in tank water: bacteria die within hours inside a stopped, sealed filter. Beyond 24 hours, change 20 % of the water daily while monitoring nitrites. Choose the replacement with our guide to internal vs external filters.
Heater down: too cold or too hot?
Too cold: the drift is slow — tropical fish tolerate 20-21 °C for a few days. Insulate the tank (blanket over the glass, lid closed), float bottles of hot water, refreshed regularly, and replace the unit. Too hot is the absolute emergency: a thermostat stuck on heat pushes the water to 32-35 °C. Unplug it, let the unit cool before removing it (it would shatter in open air), agitate the surface and gradually add cooler water — never more than 2 °C per hour.
How do you prevent rather than cure?
Replace the heater every 3 to 5 years without waiting for it to fail, clean the filter impeller monthly, and plug heater and filter into separate power strips. In areas with frequent outages, a small UPS (50 to 80 €) keeps a filter running for several hours. More safety guides in the travel and safety category.
Frequently asked questions
How long can fish survive without a filter?
With an air pump and no feeding, several days. Without any aeration, a stocked tank can run out of oxygen overnight, especially in warm water.
Is an overnight power cut serious?
A few hours are harmless in a reasonably stocked tank. On waking, restart the filter and check the temperature before anything else.
Are two heaters better than one?
On large tanks, yes: two units of half the wattage back each other up, and if one runs away it heats the water half as fast.
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.