Natural Pond Handbook

18 · Cold-Climate & Winter Care

Illustration of a frozen pond
Ice seals the top; keep a hole open for gas exchange.

In freezing regions the pond does not shut down — it slows down. Your job is to keep a breathing hole open and avoid shocking the life inside.

Why ice is dangerous

A completely sealed ice cap traps gases from decomposition (CO₂, methane, hydrogen sulfide) under the surface. With no gas exchange, these build to toxic levels and can kill fish and crash bacteria over winter.

Keep a hole open (do NOT smash ice)

  • De-icer / heater: a floating thermostatically controlled unit keeps a small melt-hole. Cheap to run and very reliable.
  • Air stone / bubbler: moving water resists freezing; keep it near the surface, not deep (deep bubbling can supercool fish).
  • Recirculating pump: a small pump pointed upward at the surface can hold a hole in mild cold.
Never Never strike the ice with a tool or stomp it. The shockwave travels through the water and can rupture fish swim bladders and kill them. If you must clear ice, pour hot water from a pan to melt a hole.

Feeding and fish in winter

  • Stop feeding when water drops below ~10°C — fish digestion halts and uneaten food rots into ammonia.
  • Fish settle into lethargy in the deepest zone where water stays ~4°C (its densest, most stable temperature).
  • Ensure your deep zone is below the local frost line so it never freezes solid.

Plant care

  • Hardy lilies and marginals die back; leave stems until spring so nutrients recycle naturally.
  • Tropicals will not survive freezing — lift them indoors or compost.
  • Net the pond in autumn to keep leaves out (rotting leaves = winter ammonia).

Spring startup

  1. Remove netting and skim accumulated debris once ice is gone.
  2. Do a partial water change (10–20%) if the water looks stale.
  3. Restart the main pump and filtration gradually as temperatures rise.
  4. Dose beneficial bacteria to re-seed the filter after winter dormancy.
  5. Begin feeding only once fish are actively swimming and water is > 10°C.
  6. Divide and replant overcrowded baskets.
Depth rule Deep zone ≥ frost depth + 20 cm. In a region with 50 cm frost, dig to at least 70–90 cm so life survives beneath the ice.
Winterisation procedure
  1. Clean filters thoroughly in mid-autumn before shutting down.
  2. Cut back dead foliage from marginals to 100 mm above water.
  3. Remove tropical plants or accept they will die.
  4. Move pump to shallow end or raise it 200 mm off the bottom to avoid pulling in cold sludge.
  5. Switch to a surface aerator or a pond air pump.
  6. Install a pond heater or float a tennis ball to keep an ice hole open.
  7. Reduce feeding to once weekly below 8°C; stop entirely below 4°C.
Never smash ice The shockwave can kill fish by rupturing their swim bladders. Always keep a hole open preventatively with a heater or floating object.