Articles and Help from North Wales Koi Club

If your pond is unheated, there are a few things you can do to help your Koi through the worst that the winter can throw at them.

I would recommend turning off, any waterfalls or Venturi’s. Filter returns above the water level should extend so that the water returns below the surface. These add cold air, which chills the water. Keep your filter pumps running, but you can turn the UV off. Bacteria levels will reduce in the cold water, but the Koi still excrete ammonia through respiration. Continue to do your filter maintenance and water changes, but this can be done every two weeks.

Unheated ponds should be checked every day to ensure every thing is still operating normally.

Food

As the temperature falls below 15degC towards 10degC, start reducing the feeding to once every few days with a wheat germ food. This helps the slower digestion rates seen at these lower temperatures. When the temperature falls below 10degC, it is time to stop feeding altogether and let the fish go dormant. The Koi will come to no harm by not feeding, as their metabolic rate and activity drops and therefore the need for food.
This dormancy is sometimes a necessary cycle, especially for females greater than 3 years of age. The cold, dormant period forces the females to re-absorb the eggs that were not used in spawning. Otherwise the eggs will harden in place, and cause complications in future years.

Insulation

The best way of stopping the pond from chilling is to fit a cover. A cover stops
the wind chilling the water surface. The idea is to stop the water temperature
falling to dangerous levels for your Koi < 5deg C.
A pond cover can be one that is specially made to exactly fit the shape of your pond; several Koi outlets can get them made to size. All you have to do is supply a template from builder’s plastic sheeting.

The cheaper option is to make your own, with timber frame with Bubble wrap stapled to it. The secret is to make a top, front and two sides, which slope to the back; this stops water and snow from collecting on the bubble wrap and making it sag. 2” x 1” roofing battens are adequate. Double up on the bubble wrap, simple double-glazing. Do not forget to leave a gap for the gases to escape. Also bubble wrap can used for insulating pipes within the filter bay.

Water Quality

Continue to do water parameter checks frequently throughout the winter, having a cover on the pond causes condensation which can run down into the water, some believe that this condensation can lower the pH of the water. If your water is soft and the pH is at the low end of the scale, i.e. 7 - 7.2, it is quite easy to get a pH crash, with the pH dropping down to dangerous levels for your koi, so keep a check on it and your koi will over winter quite happily.