Power Outage in a Reef Aquarium: What to Do First

When the pumps stop in a reef aquarium, surface movement comes first. Fish, corals and bacteria continue to use oxygen. Start your backup aeration or a circulation pump designed to run from its battery supply. Lighting can wait. At the same time, check that the sump can hold the water draining back and that nothing is leaking.

What to check first

1. Water and electricity: Has only the aquarium lost power, or the whole home? Note the time and check water levels. Do not touch wet plugs or restart visibly damaged equipment.
2. Movement in the display: Switch on the battery air pump or suitable backup supply. You should see surface movement. A pump circulating water in an isolated filter compartment does not automatically support the display.
3. Temperature and animals: Measure the temperature. Watch for unusually rapid breathing, fish gathering at the surface and marked changes in behaviour. Improve aeration promptly if you see these signs, although behaviour alone does not prove oxygen is the cause.

Pause feeding during the outage. Also pause any carbon dosing used for nutrient control: the bacterial activity it encourages consumes oxygen too. Do not add extra treatments just in case.

No battery pump? A temporary way to help

If the power fails at night and replacement equipment is out of reach, you can still disturb the surface by hand. Use a clean cup or jug reserved for aquarium use. Scoop water from the display and pour it back in a steady stream that agitates the surface. Choose a clear area away from corals and the sand bed, and keep splashes away from sockets and equipment. You are returning the same aquarium water, not adding tap water.

Repeat this as temporary assistance while arranging battery aeration or suitable help. Stirring the water once is not protection for the rest of the night. How much movement is needed depends on the aquarium; do not wait for fish to gasp before acting. Manual agitation cannot replace continuous aeration or the water distribution provided by a working circulation pump.

Why there is no guaranteed safe number of hours

Oxygen enters the water through exchange with the air. Surface movement supports that exchange, while circulation distributes the water around the tank. Consumption continues when pumps stop. Warmer seawater also holds less oxygen than cooler water. Stocking, temperature, water volume and starting conditions all affect how quickly trouble develops. A promise that everything will be fine until morning would not be reliable advice.

Manage temperature without shutting off gas exchange

If the water is cooling: Insulate the outside of the sides and back, for example with a blanket. Keep the water surface ventilated and do not cover electrical equipment or its ventilation openings. Follow the temperature trend rather than wrapping the entire aquarium without checking it.

If the water is warming: Close curtains to block direct sun. A securely positioned battery fan blowing across the surface can provide evaporative cooling. It must not be able to fall into the aquarium. Watch temperature and water level: evaporation removes water, not salt.

Do not pour hot or cold water directly into the tank to force a quick correction. Avoid adding ice cubes too. If temperature or the animals' condition keeps deteriorating despite these steps, seek experienced help immediately and, if necessary, arrange suitable supervised holding with aeration and stable temperature.

Power is back: stay for the restart

Watch the system start. The return pump and drain need to work together again: is water actually flowing, and is there enough water around the pump? Then check circulation in the display. Check heating only at the correct water level; the heater must be immersed as its instructions require. Do not raise its temperature setting to make a cooled tank warm up faster.

Restart the skimmer under observation once the sump has returned to its normal operating level. A high sump level can make it overflow. Keep automatic top-up paused while water levels are changing. Once return flow is stable, check the sensor and water level before enabling it again, and watch the first refill.

Check the clock, programme and any catch-up functions on dosing pumps. Do not blindly replace missed doses. Measure temperature and salinity first. If biological filtration was disrupted for a prolonged period or animals look affected, also check ammonia/ammonium and nitrite using tests suitable for seawater. Our water tests and water-parameter checking guide can help you choose the measurements. Continue observing the aquarium even after a first normal result.

What to prepare before the next outage

Test under supervision which aerator or pump actually runs on your backup, and how long it lasts with your battery. A large watt-hour figure alone does not answer that question. Label essential plugs, maintain battery charge and leave a short printed instruction sheet with a contact person beside the equipment. Anyone looking after the aquarium should be able to find and understand it without internet access.

Afterwards, record the outage duration, temperature trend and any equipment that failed to restart reliably. That tells you what needs improving in your own setup.