I have been getting customers ringing me and coming into the shop asking for advice about their fish behavior and condition in this cold weather.
One of the most important pieces of equipment you should have is a Pond Thermometer and I am constantly surprised that some pond keepers do not own one. A simple pond Thermometer costs as little as £6.
When the water temperature drops to 15c a Koi’s immune system starts to slow down. At 10c their immune system is only running at 10% but both good and bad pond bacteria (Aeromonas) are still running at 60% efficiency, which puts your fish at risk. This temperature range is commonly referred to as “Aeromonas Alley”. If your water quality is not good, it is within this temperature range that you could experience problems with your fish.
At 10c STOP feeding your fish, even if they are looking for food.
The lowest temperature that your Koi can comfortably survive in is 4c. Any lower than this temperature and the weaker Koi may start to develop internal problems that they may never recover from. At 1c your fish will start to die.
Another side effect of fish that have been in a torpor state at low temperatures is that they may rub their undersides on the pond bottom causing skin lesions and ulcers.
Heating your pond in winter is the best solution, but this is not necessary if you follow some simple precautions. Switch off fountains and waterfalls, keep the filters running but direct your return feed directly into the pond, preferably just below the surface. Keep aerating the pond but at a reduced rate and if possible position the air stones at half the pond’s depth. Cover the pond but always leave an air gap for escaping gasses.
Covering your pond is essential if the pond depth is not greater than 4 feet (122cm). For small ponds of around 4550L (1000 UK g) to 9100L (2000 UK g) you may find that the introduction of a metal sleeved aquarium type heater may keep your pond at 4c or even above.
I hope you find this article of some use and it does not come too late to save your precious Koi from the harsh trials of winter.
Mark Grist
Koi Hut Manager