How to Prepare Your Pond for Winter (and Restart It in Spring)

A garden pond does not need to be shut down for winter, but it does need a little preparation. As the water cools below about 10°C, pond fish slow right down, their digestion becomes sluggish and the filter's bacteria become far less active. A few jobs in late autumn will protect your fish, keep the water healthy under ice and make the spring restart much easier.

This guide walks through what to do, in the order to do it, plus the mistakes that cause most winter losses.

1. Clear leaves and debris before they sink

Falling leaves are the biggest winter risk to most ponds. Leaves that sink rot slowly through the cold months, releasing ammonia and phosphate and using up oxygen just when the pond is least able to cope. By spring they become a layer of sludge that feeds green water and blanketweed.

• Net the pond early. Stretch a pond cover net over the water before the main leaf fall, pegged down and lifted slightly in the middle so leaves roll off. A net such as the Blagdon Pond Cover Net 6m x 3m also keeps herons away while fish are slow and vulnerable.
• Skim the surface regularly. Remove floating leaves before they become waterlogged.
• Vacuum the bottom once. A single autumn clean with a pond vacuum such as the Oase PondoVac 4 or Eheim Pond Vac 40 removes the sludge already on the floor. Avoid stripping the pond completely; a thin layer of mulm is harmless.

2. Cut back marginal and oxygenating plants

Trim dying foliage from marginal and deep-water plants so it doesn't collapse into the water. Cut water lily leaves back as they yellow. Leave a few hollow stems, such as rushes, standing: they help gases move through ice. Tropical floating plants such as water hyacinth will not survive a UK winter and should be removed.

3. Change how, and how much, you feed

Feeding is where most winter problems start. Fish cannot digest food properly in cold water, and uneaten or undigested food pollutes the pond while the filter is barely working.

A low-protein wheatgerm stick such as Tetra Pond Wheatgerm Sticks is designed for this transition period. You'll find other options in our pond winter foods range. Use a simple floating pond thermometer, and go by water temperature rather than the calendar. Healthy fish build reserves over summer and come to no harm from not eating for several months.

4. Decide what to do with your pump and filter

There are two sensible approaches, and the right one depends on your pond and fish.

Keep the filter running (recommended for koi and fish ponds)

Running the filter all winter keeps the bacterial colony alive, so it's ready to work when feeding resumes. If you do this:

• Lift the pump off the very bottom so it doesn't pull up the warmest water where fish rest. A shelf about 30–45cm below the surface is ideal.
• Turn waterfalls and streams off, or reduce flow. Moving water over cold stone chills the whole pond.
• Clean the filter media gently in pond water, never tap water, so the bacteria survive.

Switch off for winter (small wildlife or lightly stocked ponds)

If you switch off, remove the pump, clean it and store it in a bucket of water so the seals don't dry out. Take UV clarifier bulbs out, as they aren't needed in winter and you'll want a fresh bulb in spring anyway. Drain any external filter boxes and pipework that could freeze and crack.

Whichever you choose, switch off the UV clarifier for winter. Algae growth stops in cold, dull weather, and the bulb loses strength every hour it runs.

5. Keep oxygen up and an air hole open

Ice itself rarely harms fish. The real danger is a sealed surface that traps toxic gases from decaying material below and stops oxygen getting in. Keeping even a small area open makes a big difference.

• Run an air pump. A pond air pump such as the Evolution Aqua Airtech 75 or Airtech 95 adds oxygen and the rising bubbles help keep a patch of surface clear. Place the air stone on a shelf, not the deepest point, so the warmer bottom layer isn't mixed. Use sinking pond airline so the tube stays out of sight and out of the way.
• Never smash the ice. Shock waves through the water can stun or kill fish. If the pond freezes over, stand a pan of hot water on the ice and let it melt a hole gently.
• A floating ball helps. A tennis ball or float on the surface can be lifted out to leave a hole after a light frost.

6. Check water quality before the cold sets in

Test your pond water in October or November while fish are still active. If ammonia, nitrite or nitrate is high, do a partial water change of around 10–20% using treated tap water before temperatures drop. Always use a pond dechlorinator with tap water. Going into winter with clean water and a reasonable stocking level is the best protection you can give your fish.

7. Spring restart checklist

When the water climbs back above 8–10°C, usually in March or April, it's time to wake the pond up.

1. Reinstall and restart the pump and filter if they were stored. Check seals, impellers and pipework.
2. Add a filter starter such as Evolution Aqua Pure Pond to rebuild the beneficial bacteria. See our pond filter starters.
3. Fit a new UV clarifier bulb before algae season starts.
4. Remove the net and vacuum any debris that has settled.
5. Restart feeding slowly with wheatgerm food, moving to normal food once the water is consistently above 15°C.
6. Watch fish closely for the first few weeks. Spring is when fish are at their weakest and parasites are most active.

Common winter mistakes to avoid

• Feeding fish that are no longer able to digest food properly.
• Leaving leaves to rot on the pond floor.
• Running a waterfall all winter and supercooling the pond.
• Breaking ice with force.
• Cleaning filter media in tap water and killing the bacteria.

Your winter pond kit

Most ponds need only a net, a suitable food and a way to keep oxygen up. Browse our pond cleaning, pond pump and winter food ranges, or read Pond Cleaning Explained for a year-round maintenance routine. If you have questions about your own pond, our team is happy to help.