We live in a rural area, and our electricity supply can sometimes be erratic, causing power outages. Our son has just set up a tropical fish aquarium - what should we do in the event of a power cut ? 

When there is a loss of power, not only will the water temperature start to decline in the aquarium, but aeration of the water will cease. The biological efficiency of the filter will be compromised, and the lights will go off as well.

Perhaps surprisingly, the most serious long-term effects may be on the filter, but immediately, the first thing to do when the power goes off is to cover the aquarium, and so minimise the heat loss from the water. A thick blanket or even a duvet can be used for this purpose. 

Provided that the room itself is reasonably warm, then the temperature should only decline slowly, enabling the fish to adjust accordingly, and hopefully, before long, the power supply will be restored. 

What to do afterwards

Start by removing the cover as it will be a fire hazard if it is left in place once the system is operating again. Then reconnect the power supply. The heaterstat will switch on, and the water will warm up slowly, minimising the stress on the fish. The other equipment such as the filter and lights should start working again too. 

For a week or so afterwards, carry out regular water quality checks, particularly for ammonia and nitrite, to see how badly the beneficial bacteria in the filter have been affected. Be prepared to carry out more frequent water changes during this period if levels have risen, ensuring that you have an adequate supply of water conditioner available for this purpose.

It will also be worthwhile adding a beneficial bacterial culture to the filter. This should help to regenerate the bacterial population here more rapidly than leaving the existing bacteria to do so on their own.

Try to avoid stressing the fish, and do not feed them immediately, as they are unlikely to eat straightaway, and any uneaten food will simply pollute the water. Watch for stress-linked problems in the fish, notably fungus, and be prepared to treat this as soon as possible. Hopefully though, they should be fine, especially if they are well-established in their surroundings.