The short answer
During a power cut, your two priorities are keeping the filter media wet and keeping the tank warm. Fish themselves are surprisingly resilient — the bigger risks are your beneficial bacteria dying if the filter media dries out or goes stale, and the tank losing heat. Handle those two things and most tanks ride out a blackout with no lasting harm.
Protect your beneficial bacteria
The bacteria in your filter need oxygen and moisture. When the pump stops, water stops flowing through the media and it slowly goes stale.
- Keep the media wet — leave it in the filter, or move it into a tub of tank water.
- For a long outage, stir or pour that water occasionally to add oxygen.
- When power returns, restart the filter promptly. If it was off for many hours, a stale filter can release ammonia, so test the water afterwards.
Keep the tank warm
Water loses heat slowly, which works in your favour. Help it along:
- Insulate the tank — wrap it in blankets, towels or foam and close the lid.
- Don’t open the tank unnecessarily; every peek lets heat escape.
- Float sealed bottles of warm water for a gentle top-up if the room is cold (never pour hot water directly in).
Keep oxygen up
Without the filter agitating the surface, oxygen can slowly drop, especially in a stocked tank. A battery-powered air pump is the single best blackout tool — many aquarists keep one on hand. Failing that, gently cup and pour tank water back in every so often to break the surface. See our air pumps hub.
After the power returns
Restart equipment, then watch the water. Test over the next few days in case the filter lost bacteria — see how to cycle an aquarium if you see an ammonia spike, and keep up your water testing routine. If fish look stressed afterwards, see how do I know if my fish is sick?