How do I look after a pond in freezing weather?

A frozen pond kills fish by trapping gases under the ice, not by the cold itself. Keeping one small area open, without smashing or boiling anything, is the whole job.

Difficulty: beginner
Time: 20 min
Read: 5 min
Safety: caution

Short answer

Do not break the ice. Stand a pan of hot water on the surface until it melts a hole, or float a ball overnight and lift it out in the morning. One open patch lets gases escape, which is what fish actually need. Turn fountains off, leave the pump running deep or take it out, and stop feeding once the water is below about 8 degrees.

Fish and pond life survive UK winters perfectly well under ice; what kills them is a sealed surface. Decomposing leaves and mud at the bottom give off methane, carbon dioxide and hydrogen sulphide all winter, and with the lid on, those build up while oxygen is not replaced. A single hole in the ice fixes it. Smashing the ice does not — the shock travels through water and can rupture a fish's swim bladder.

Safety

Never walk on, kneel on or break ice on a pond by hitting it. Ice over a garden pond is rarely strong enough to hold a person, and a shock wave from a blow can kill fish outright. Keep children away from a frozen pond entirely — a garden pond is deep enough to drown in and the ice looks reassuring from a distance.

What you'll need

A saucepan of hot water
A floating ball, or a pond de-icer (optional)
A net or rake for surface leaves
A pond thermometer (optional)

Step by step

Melt a hole rather than making one.Stand a saucepan of hot — not boiling — water on the ice and let it sink through. Take the pan away and leave the hole. Ten minutes of standing there is all it takes.
Or float a ball before the freeze, and lift it out after.A football or a plastic bottle part-filled with water sits in the surface as it freezes. Lift it out in the morning and there is a ready-made hole. It is the least effort of any method.
Turn off any fountain or waterfall in hard frost.A fountain chills the whole pond by circulating surface water and mixing away the slightly warmer layer at the bottom where the fish are. It also risks freezing the pipework and cracking a pump housing.
Decide what to do with the pump.Either lower it so it runs slowly near the bottom and keeps a small area moving, or lift it out, clean it and store it in a bucket of water somewhere frost-free so the seals do not dry out. Do not leave a pump sitting idle in freezing water.
Stop feeding once the water drops below about 8 degrees.Fish digestion effectively shuts down in cold water. Food they cannot digest rots in the gut and in the pond, adding to exactly the gases you are trying to vent.
Get the leaves out before winter, not during it.Autumn leaf fall on the bottom is the source of most of the gas. Net the pond in autumn, or scoop leaves off the surface weekly, and pull decaying vegetation out before the first freeze.
Leave a pond de-icer to do it if you are away.A low-wattage floating de-icer keeps a small area open without heating the pond. It must run from a proper outdoor-rated socket with RCD protection, ideally installed by an electrician.
Do not tip boiling water straight in.Boiling water poured onto ice or into the pond shocks the water underneath. Hot water in a pan on the surface transfers heat gently and does not mix with the pond at all.

Tips

A pond that is at least 60cm deep somewhere will not freeze solid in a normal UK winter, and that depth is the single best protection for fish and frogs.
If the pond has frozen over for several days and you find dead fish or a strong smell when you open a hole, that is gas build-up. Open a hole and leave it, but do not attempt a big water change in freezing weather.
Frogs overwinter in the mud at the bottom and are more vulnerable to a sealed surface than fish are. The same open hole protects both.
Marginal plants in baskets can be moved into deeper water for the winter so their crowns are below the ice line.

Common mistakes

Breaking the ice with a hammer, brick or spade — The pressure wave through the water can kill or injure fish even where the ice itself is thin. It also achieves nothing that a pan of hot water does not achieve more gently.
Running the fountain through a cold snap to stop it freezing — It chills the whole pond and destroys the temperature layering the fish depend on. Turn it off and keep one small hole open instead.
Carrying on feeding through winter — Uneaten and undigested food decomposes, which uses oxygen and generates the gases that make a sealed pond lethal.

If it doesn't work

Hole freezes over again within hours

Cause: Very hard frost, or a hole made too small — Fix: Use a floating ball or a thermostatic pond de-icer so it stays open without daily attention. In an extended freeze, one visit a day with a pan of hot water is enough.

Fish at the surface gasping when the ice clears

Cause: Low oxygen, usually from a long sealed period or decaying matter — Fix: Open more of the surface, remove decaying leaves you can reach, and run an air pump or a low fountain once the hard frost has passed. Do not do a large water change in cold weather.

Cracked pond liner at the waterline in spring

Cause: Ice expanding against a vertical edge — Fix: Float a ball or a bundle of polystyrene through the freeze in future to absorb the expansion. Repair the liner with a proper liner patch kit once the pond is warm and dry at the crack.

Questions people ask

Should I drain the pond for winter?

No. A drained pond kills everything living in it, and an empty liner is vulnerable to frost and UV damage. Ponds are meant to freeze at the surface; they are not meant to be emptied.

Will pouring salt or antifreeze help?

Never. Both are toxic to fish, amphibians and plants. There is no chemical you should ever put in a pond to stop it freezing.

Do I need to worry about a wildlife pond with no fish?

Much less. Frogs and invertebrates cope well, though an open hole still helps in a long freeze. The pond that needs attention is a stocked, deep-mud, leafy one.