Should you turn off your pond filter in winter?

It's the big debate every winter, on forums and in stores: should you turn off the filtration or leave it running? Both sides have good arguments, and neither is right for every pond. Since 1998, we have found that the result mostly depends on two simple questions: is your filter protected from frost, and are there fish in the pond?

Decide in 30 seconds · The arguments of both sides · Temperature guidelines · Leave it running · Turn it off properly · The equipment · Frequently asked questions

Decide in 30 seconds

Why the debate returns every year

Koi keepers insist on the life of the filter: beneficial bacteria take weeks to establish themselves, and a complete shutdown deprives them of water and oxygen. Others point out that in winter the fish eat very little, produce little waste, and that poorly adjusted circulation cools down the entire pond. Both are right, but not about the same pond.

What happens below the surface

Fish are cold-blooded animals: their bodies take the temperature of the water. Below about 5 °C, they enter a state of lethargy and remain almost motionless at the bottom. Since water is densest around 4 °C, this heavier water remains at the bottom if nothing mixes it. This is their resting zone, and that is why we recommend a depth of at least 80 cm.

What to do according to the water temperature

It is the water temperature that decides, not the calendar or the daily weather. A floating thermometer is enough to monitor the trend over several days.

If you leave the filtration running

• Raise the pump about 30 cm above the bottom, so as not to disturb the fish or stir up the deepest zone.
• Turn off the waterfall: it can freeze and cause water to overflow out of the pond.
• Reduce the flow if your pump allows it. The filter needs a flow, not a torrent.
• Insulate the filter and pipes exposed: they are the first to freeze.
• Unplug the UV: the light is very weak at this time in Belgium and France.

All the pump settings are detailed in our guide Pond pump in winter: leave it running, raise it or turn it off?

If you turn off the filtration: the procedure to follow

1. Turn it off before the first hard frosts.
2. Completely empty the filter and pipes, then clean the filter.
3. Disassemble the UV device, because the quartz is fragile. See our article wintering your pond UV lamp.
4. Clean the pump. If the pond is deep enough, put it back in. If you take it out, keep it in a basin of water so that the seals do not dry out.
5. Check that the air pump is working and that the diffusers are clean.
6. Raise the diffuser a little so as not to mix the cold surface water with the warmer water at the bottom.

The opening in the ice, the point not to neglect

If you turn off the filtration, always keep an opening in the ice. It allows gases from decomposition to escape and maintains exchanges with the air. An air pump with a diffuser or a floating heater does this job. For a pond less than 70 cm deep, an anti-freeze dome with a small heater is enough.

The equipment that saves you from unpleasant surprises

Four solutions chosen by our experts, according to the size of your pond:

Find all our selection in the winter protection section. Free delivery from 149 €.

And in spring?

When the water rises permanently to about ten degrees, restart the filtration at a moderate flow, before resuming feeding. If the filter was turned off, its bacteria must be reconstituted: an addition of bacteria for ponds helps it restart faster.

Frequently asked questions

Should you turn off the UV in winter?

Yes, you can unplug it: the brightness is very weak at this time in Belgium and France. If it is at risk of freezing, empty it and disassemble the quartz.

What depth is needed to keep fish in the winter?

We recommend at least 80 cm, so they have a stable area to rest. In a shallower pond, plan for an anti-freeze system.

Will my filter die if I turn it off?

Bacteria deprived of circulating water largely perish. You will need to restart the filter gradually in spring.

Should you feed the fish in winter?

Below 5 °C, they are in a state of lethargy. Sturgeon remain the exception and must continue to be fed. On beautiful days, some owners give a little floating food.

Why keep an air pump if the filter is turned off?

It maintains an opening in the ice, lets gases out and continues to oxygenate the pond.

Also read

• Pond pump in winter: leave it running, raise it or turn it off?
• Winter protection of the pond: pump, filter, UV and plants
• Wintering your pond: preparing the fish