The pond has a power failure
Will my fish come to any harm?
If your fish are used to having moving water from a filter, fountain, or waterfall, then the failure of the pump might cause problems. This movement can help to aerate the water and increase its oxygen content. Problems are more likely in warm weather when fish, pond wildlife, and filter organisms are using more oxygen from the water. Problems are less likely in winter as cold water usually contains more dissolved oxygen to start with. Fish also slow down and use less oxygen in cold weather. www.watergardensolutions.co.uk
As long as your pond is not overstocked, a power cut of an hour or so in summer, or of a day or so in the depths of winter, is unlikely to cause immediate problems.
What if the power cut starts to cause problems?
The oxygen levels in the water will start to drop. The fish may start mouthing at the surface of the water. Organisms inside a biological filter will stop functioning correctly. If the problem persists, fish (especially larger fish, or oxygen sensitive fish like orfe) may start to die, and the filter organisms may die back.
What can I do?
Stop feeding the fish.
The oxygen levels in the water can be increased by maximising the water surface exposed to the air. Remove excess/yellowed floating leaves from waterlilies and net out excess quantities of floating aquatic plants (like duckweed).
Spray in tap water. A jetted stream of tap water from a hose directed onto the pond surface will both aerate and circulate the pool water as well as slowly changing some of the water. If you end up changing more than 5-10% of the water you should use a suitable water conditioner to reduce the risk of damage from the chlorine in the tap water
If you have a raised box filter with a drain tap, drain some water from the base to waste, to flush out the worst of the debris.
Ponds and filters that are well maintained will have fewer problems in the event of a power cut. By routinely removing excess wastes and debris from the pond and filter, there will be less waste to use up the oxygen in the water. However, don’t attempt to disturb the debris on the pool base in the middle of a power cut, as stirring this up will only use up valuable oxygen.
When the power restarts
Ideally, drain stagnant water from any filter system to waste, rinsing any dirty filter media in the old pond water first. Then restart the filter. Check than non-latching RCD/trip-switches are turned back on. If the power has been off for some time, the biological filter organisms could take three to six weeks to recover. Feed fish very lightly during this period and do not add any new fish stocks.