How to Cope With Power Cuts in a Fish Tank

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Our aquariums are artificial habitats and as such rely on various bits of technology to function properly. Our heaters, filters, and lights are all substitutes for natural life-giving parameters in nature, and are essential for the health of our aquariums inhabitants.

Power cable unplugged

When power cuts occur, for whatever reason, these life support systems are switched off and detrimental changes start to happen within a few hours. To make sure your fish survive unharmed from power cuts, it is important to understand exactly what happens when these systems shut down, and how to minimise any potential damage both during and after a loss of power.

Tropical fish tank heating

Tropical aquariums are normally heated by a heater-thermostat unit set to between 24-27C (75-80F), ambient room temperature however is usually around 18-20C (64-68F) When a heater fails, the aquarium water inevitably begins to fall towards ambient room temperature.

This change is gradual however, and most fish will cope well with a gradual drop and rise over a 24-hour period. The fish may become sluggish and behave differently, but should recover quickly. Many fish experience changes like this in nature, so it is not a major problem unless you keep delicate species.

Problems can occur afterwards due to disruptions to other areas of the aquarium, such as filter bacteria, although these are dealt with later on. If you do keep delicate species, and the heater-stat is off for more than a couple of hours, the best you can normally do is to pack the aquarium sides and top with insulating material such as polystyrene, foam, or bubble wrap to reduce the rate of heat loss.

As an improvisation, use anything to hand such as blankets or covers, and pay particular attention to the aquarium hood, where most heat is lost.

Never try to introduce another heat source (except as mentioned later on), such as using a heater near the aquarium, this may produce sharp and uneven temperature changes, and may even heat the water above its normal temperature. A gradual steady loss of temperature is far preferable than a continually fluctuating and uneven temperature.

Aquarium lighting

Loss of lighting over a 24-hour period is a disruption, but should not affect the health of your plants or fish to any great degree. If you have a very heavily planted aquarium, in dark conditions the plants will take oxygen out of the water, so it might be wise to look at methods of aerating the water during longer periods of light loss. Otherwise, apart from a little confusion, your aquarium will recover quickly after temporary loss of light.


This is the most important piece of equipment in any aquarium, and will cause the most problems in a power cut. Filters depend on bacteria that live within them, and those bacteria depend on a continual supply of oxygen obtained from the water flowing through the filter. Once that water flow stops, the remaining oxygen is used up, and after a period, the bacteria begin to die, followed by a release and build-up of toxins.

When the filter is eventually switched back on again, all the de-oxygenated and polluted water flows back into the aquarium, and because of a loss of bacteria, the filter does not remove waste products present in the aquarium or produced over the following days, or weeks.

When this happens, aquarium conditions worsen for many days after the power loss, and fish health problems are the result, often occurring days later.

For long power cuts, there is the added problem of oxygen loss in the aquarium, since the outflow of water from the filter is normally the main source of surface agitation, where oxygen enters the aquarium. If the power/filter is switched off for a short period (say 15 minutes), there should be no major problems, if longer, follow the advice below.

What to do with your fish tank in a power cut

Depending on the length of power loss, there are various things you can do to prevent any major problems with your aquarium, and avoid any loss of livestock. Many of these are to do with the disruptions caused for days afterwards rather than the actual event of a power cut.

One major factor that is often overlooked is feeding, if the aquarium is disrupted by power cuts, the normal processes which break down waste products from your fish may not be working correctly. Therefore, it is important to minimise any waste production by cutting down or avoiding feeding during and after a power cut.

Most importantly - be prepared! If you live in an area where power cuts are regular, it would be wise to invest in a generator, or at the very least a battery powered air pump. Have the number of your electric company handy, and ring them to see how long the power will be off for, this will give you an idea of whether you need to make any preparations.

0-30 minutes

For short events like this, there is no need to do anything and your aquarium will carry on as normal once the power is back on, however it would be wise to avoid feeding for at least 12 hours after the power loss, just to be safe.

30-60 minutes

If you have an external filter, remove all the media and give it a gentle clean in aquarium water. Rather than return the media to the filter, place it directly in the aquarium. Most external filters contain baskets or sections, which can be removed with the media inside.

Placing the media baskets directly in the aquarium should allow any vital filter bacteria to survive on oxygen from the aquarium water. If left in the filter, the bacteria will die, the water trapped in the filter will become toxic, and when the system is switched back on, the toxic water will be returned to the aquarium.

An alternative to removing the media is to 'drain' the external filter every 15 minutes or so, which can be easily done by placing the outlet pipe in a bucket below the filter level, and allowing water to siphon through. This will replace the water in the filter with new (oxygenated water) from the aquarium, keeping the filter bacteria alive. Do not feed during the power loss, or for 24 hours afterwards.

1-2 hours

Same as above, but apply the same technique to internal filters, removing the media and placing it directly in the aquarium.

3-4 hours

Longer outages like this can begin to cause serious problems and you will need to look at both temperature and oxygen levels, as well as placing filter media in the aquarium as mentioned above.

If your house is heated by non-electrical means, then raise the room temperature as much as possible; this will prevent the aquarium cooling down too much. Alternatively, if you have a source of hot water, then float containers of hot water in the tank, changing them as they cool. Do not add hot water directly to the aquarium, since it may contain toxins from heating systems.

Oxygen levels are a little trickier to control unless you have a battery powered air-pump, in which case this should be used near the filter media so that both the media and the aquarium water is oxygenated. Observe your fish carefully and look out for signs of oxygen deficiency, including gasping at the surface or rapid breathing. As a last resort, take some water from the aquarium, shake it vigorously, and return it to the aquarium (you may need to do this several times). Blowing through an airline or tube into the water is not ideal, since the air we exhale is low in oxygen, and high in carbon dioxide.

More than four hours

If your electric is off for more than four hours and you do not have a battery-powered air pump, or are unable to keep the aquarium warm, it might be wise to invest in a petrol powered generator to provide a power source for your aquarium. These can be bought from most DIY stores, and might save the lives of your fish. Make sure you run the generator outside, or away from the aquarium to avoid pollutants entering the tank.


Unfortunately, the problems associated with aquaria and power cuts are not over once the power is back on. If the loss of power was for anything more than an hour, it is very likely that the aquariums population of filter bacteria has been disrupted, meaning that waste will not be processed in the normal manner. Therefore, after a power cut you should keep feeding light, and monitor the aquarium daily for any signs of ammonia or nitrites, in the event of which you can stop feeding temporarily, and apply the relevant actions to return conditions to normal.

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