Heater or filter failure: the emergency kit that saves the aquarium
Published by Planète Pets · managing editor: Clément Lacaille · how our guides are written
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.
Contents · 5 sections
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.
In our shop, for this use: 6 cm nano air diffuser with suction cups for aquariums (€19.90) · the whole shop
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.
