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Ice is not always a problem
Cold water usually has a good amount of oxygen dissolved in it. Any fish and wildlife, sluggish in the cold, will also be using up less oxygen. If the pond entered the winter in a healthy state, without an excessive build up of decaying leaves or debris on the base, and with moderate stocks of fish, there shouldn’t be problems for pondlife. Pond inhabitants should be able to cope with the surface being sealed by ice for two or three days without coming to any serious harm. copyright www.watergardensolutions.co.uk
During prolonged cold
If there is no water movement, and the surface freezes solid for more than a few days, problems can arise for pondlife. The oxygen levels can start to fall, and dissolved waste gases can start to build up in the water, poisoning wildlife. This is especially a problem where the pond is heavily stocked with fish or frogs; has a lot of debris on the base; or has snow lying on the ice - stopping light from reaching underwater plants.
If your filter/cascade pump can be kept running, this should provide sufficient circulation and aeration, even if the majority of the surface is sealed by ice. Note that Koi can be weakened by long periods at very low water temperatures (< 4 degrees C), and extra precautions may be required.
Pumps, filters and UV units are less likely to suffer frost damage if they are kept running. However, after prolonged freezing (lasting many days), waterfalls and filters can eventually ice up. This can sometimes cause water loss, and you may be forced to turn pumps off. If you do turn off pumps, fully drain exposed units, otherwise water inside the equipment will freeze and could split the casings or crack UV sleeves. Any biological filter which is turned off or drained in the winter, can take three to six weeks to become active again, once they are restarted in the spring.
If you have no water-pump
An option is to use one of the small floating electric pond-heating units or a small external air-pump (feeding an airline to a bubbling airstone). This keeps an ice-free area allowing oxygen in, and waste gases out, at the surface. The open water will also be valued by birds and other garden wildlife. Polystyrene dome units can keep a vent in the ice for a while, but eventually freeze up. A floating ball is only of use once removed from the ice, and cannot protect concrete ponds from damage.
If you don’t have any of this equipment, keeping a hole open in the ice is still beneficial. Don’t smash the ice, as the shock waves can upset fish, and jagged ice can damage liners. Melt a hole by carefully pouring on hot water from a kettle (avoiding pondlife), or resting a pan of hot water on the ice. Check that the hole is kept open at least twice a day. Very large ponds may require multiple holes.
Pond damage?
Flexible liners and plastic/GRP moulded ponds are rarely damaged by ice pressure, but concrete ponds, stone/terracotta features, and exposed pipework & valves are more vulnerable. Insulating covers may help a bit. Vulnerable ceramic containers are better drained or moved to a sheltered area.
After the ice has gone
If the pond smells stale after the ice has melted, carry out a partial water change using tap water conditioner if necessary.
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