Pond pump in winter: should you keep it running, raise it, or turn it off?

Every year, as soon as the first frosts arrive, the question comes up at the store: “Do I turn off the pump or leave it running?” And every year, the honest answer is the same: it depends. Not to dodge the question, but because a decorative pond without fish, a natural pond, and a koi pond with a gravity-fed filter do not have the same needs at all. In winter, the point is not to keep the pump running at all costs, but to avoid mixing the warmer water at the bottom with the icy surface water. From there, three options emerge.

Why the pump's position matters more than whether it is running

Water reaches its maximum density at around 4 °C. In winter, this slightly less cold water sinks and accumulates at the bottom, while the surface cools and eventually freezes. This is what limnologists call winter stratification, as described in detail by Wetzel. For koi and goldfish, which are sluggish and motionless, this bottom layer provides a refuge.

A pump placed on the bottom, which draws in this water and sends it back to the surface via a waterfall, does exactly the opposite of what we want: it homogenizes the entire pond to the ambient air temperature and accelerates ice formation. In a koi pond, this is a common mistake, and sometimes one with serious consequences.

The three options, case by case

Option 1: keep it running, but differently

If your pond contains fish and has a mature biological filter, stopping the pump for three or four months deprives the filter bacteria of water and oxygen. In spring, you are almost starting from scratch, precisely when the fish start eating again. That is why most koi keepers keep the filtration running in winter.

But adjustments are needed. The waterfall, stream, and jets are switched off or bypassed: the water must return to the pond below the surface, without falling. A submersible pump is placed on a stand, brick, or block so that it draws water from mid-depth rather than from the bottom. And whenever possible, reduce the flow rate: the filter needs a flow, not a torrent.

A pressurized filter or an above-ground gravity filter remains the most exposed component: it is the one that freezes first, not the pond. Insulate it, partially bury it, or protect it with a suitable cover. The UV units can be switched off during the cold months: they are of little use when algae are dormant.

Option 2: stop it and store it properly

In an ornamental pond without fish, or for a pump that only powers a fountain, stopping it is the simplest choice. Do this before severe frosts. Remove the pump, clean the grille and rotor, then store it in a frost-free place according to the manufacturer's instructions; some recommend keeping it submerged in a bucket of water to prevent the seals from drying out. Drain the pipes, filter, and UV housing: water that freezes in a quartz sleeve or filter body can crack it.

If the pond contains a few fish but has no filter, stopping the pump is acceptable provided you keep an opening in the ice so that gases produced by decomposition can escape.

Option 3: replace the pump with gentle aeration

This is a very common middle-ground solution: the filtration pump is stopped or slowed down, and a small air pump takes over. The diffuser is then placed at a shallow depth, around twenty centimetres below the surface, never at the bottom. The bubbles keep an ice-free area open and maintain gas exchange without breaking up the protective layer. Be sure to protect the air pump from moisture and frost, and place it higher than the water level.

The mistakes we see every winter

• Leaving the waterfall running all winter in a shallow pond.
• Turning off the pump during a cold snap without draining the hoses or filter.
• Place the aerator diffuser at the deepest point.
• Forgetting to clean the pump before storing it: limescale and sludge harden.

Also read

For proper aerator installation: Where should I place the air pump in my pond?.

READ THE ARTICLE

Frequently asked questions

— Can a pump freeze in the pond?

A submersible pump at a suitable depth will not freeze as long as it remains under the ice. The hoses, filter, and components outside the water are vulnerable.

— Should I turn off the waterfall in winter?

Yes, in a pond with fish. A waterfall cools the pond considerably and can form ice blocks that divert water outside the liner.

— Will my filter die if I stop the pump?

Bacteria deprived of circulating water largely die off. In spring, restart the filter gradually, possibly adding bacteria.

— When should I restart the pump in spring?

When the water rises steadily to around ten degrees, starting with a moderate flow, before resuming feeding the fish.

In summary

• The winter challenge: do not mix the warmer bottom water with the cold surface water.
• Fish and biological filter: keep the pump running, raised up, without a cascade, with reduced flow.
• No fish: clean the pump and store it in a frost-free place; drain the hoses and filter.
• Intermediate solution: a shallow aerator or floating de-icer.
• Decide before severe cold sets in, never during a hard freeze.

Last updated: September 29, 2026.