When aquarium power fails, focus first on oxygen and temperature. Use a battery air pump if available, reduce heat loss in cold weather or solar gain in hot weather, and avoid feeding because digestion and waste increase oxygen demand. Lighting can remain off. If a canister or other closed filter has been stagnant for many hours, inspect and service it before blindly pumping stagnant water back into the aquarium.
A power cut removes several support systems at once, but they are not equally urgent. Fish can tolerate darkness; they cannot tolerate falling oxygen indefinitely.
Priority one: oxygen
A battery air pump is one of the most useful emergency items for an aquarium. If you do not have one and fish begin gasping, manually agitating the surface can temporarily improve gas exchange.
Heavily stocked and warm aquariums generally lose oxygen margin faster than lightly stocked cool tanks.
Priority two: temperature
In cold weather, close windows and insulate the sides and top while still allowing necessary gas exchange and safe access. In hot weather, keep direct sun off the tank and avoid trapping heat.
Do not add very hot or very cold water suddenly.
Stop feeding during the outage
Fluval advises not feeding during a prolonged outage. Fish generate less waste and oxygen demand when they are not being fed, and healthy adults generally tolerate a short missed meal better than poor water quality.
Resume normal feeding once the system is stable.
Think about the filter
Without flow, oxygen inside closed filters falls. After a long outage, trapped organic matter can become unpleasant or potentially harmful.
For a prolonged outage, follow the filter manufacturer's restart guidance and consider rinsing dirty media gently in conditioned or aquarium water before restoring flow.
When the power returns
Restart aeration and filtration, confirm the heater and thermostat are working, and check temperature. In heavily stocked tanks, test ammonia and nitrite after a significant outage.
Watch fish behaviour closely for several hours.
Frequently asked questions
Should I feed fish during a power cut?
For a significant outage, it is generally better not to feed until filtration and oxygenation are restored.
Do aquarium lights matter during an outage?
They are a low priority compared with oxygen, filtration and temperature.
Should I restart a canister filter immediately after a long outage?
Inspect and service stagnant closed filters if the outage was prolonged rather than automatically pumping stagnant water back into the tank.
Sources and further reading
This guide separates general aquarium arithmetic from fish-welfare decisions. For species-specific limits, always check the needs of the fish you intend to keep.