Temperature & heating

The year

The water follows the seasons. These are the natural water temperatures through the year:

Spring warms from 12 to 20°C, summer peaks around 26–28°C in its middle weeks, autumn cools to 10°C and winter sits at 4–8°C.

What warmth changes

Warm water is a double-edged sword: koi eat, grow and breathe more just as the water holds less oxygen. The bacteria love it, but the extra food makes more ammonia for them to clear.

Choosing a temperature

Planned With a heater you’ll set a thermostat for each pond, anywhere from to °C. What to aim for depends on the pond:

• 15–18°C: cheap to hold, plenty of oxygen, koi still eat and grow a little, bacteria keep working. A sensible winter target for a show pond.
• 20–22°C: growth all winter, but it costs more energy, more food, more waste, and the water holds less oxygen, so you need air stones in winter too.
• Breeders: koi need a cold winter rest to come into condition for spring. A warm breeding pond spawns poorly, so leave it cold.
• Too hot: above about 26°C the oxygen gets tight. A heater won’t cool a pond. Shade, a deeper pond, air stones and a water change with cool tap water will.

Heating

Planned The numbers below are placeholders while we balance energy.

Heat leaves a pond through its surface, so surface area decides the running cost. Volume decides how long it takes to warm up and how slowly it cools.

kWh a day to hold = U × surface (m²) × (target − natural °C) × 24 ÷ 1000

kWh to warm up = 1.16 × litres ÷ 1000 × °C of lift

Water changes in a heated pond

Tap water is cold (), so a water change cools a heated pond:

new temperature = pond − share changed × (pond − tap)

A 25% change in a 22°C pond in winter drops it about 3.5°C, which stresses koi; a 10% change is gentle. The energy to warm the new water back up is added to the change’s cost. In summer the same thing cools an overheated pond.