Koi – The only magazine for practical Koi keeping advice

If you feel the winter chill is getting the better of you and your pond, fear not, Ben Helm is here to solve your icy problems…

Ice can pose a real threat to the health and well-being of a pond and its inhabitants. Natural water bodies such as lakes and ponds will tend to freeze throughout any typical British winter, with little impact on the fish or other aquatic life. So why do we try to battle with Jack Frost when he imposes on our own garden ponds?

A winter wonderland in your garden may look pretty, but your pond could be suffering, so look out

Frozen surfaces

In the short term a frozen pond (or more precisely its surface), will not present any problems for your Koi’s health. Koi and other pond fish will naturally descend to the bottom where they should be safe, away from the freezing surface. The desired depth of a Koi pond is four feet, although many shallower ponds can overwinter fish surprisingly well. The fish will have been preparing for the onset of winter over the preceeding weeks and months, with the shortening day length and dropping temperatures. A garden pond is likely to be more heavily stocked than a natural water body, and if it’s allowed to freeze uncontested for more than a few days, eventually the risk of problems starts to increase.

Gas exchange

An ice-capped pond is prevented from enjoying the normal gas exchanges between the water and the atmosphere. Even when water temperatures are low fish still continue to metabolise, requiring oxygen and releasing CO2. Bacteria and pond life release CO2 and other non-desirable volatile products when they break down a range of accumulated organic matter. These gases accumulate and can reach levels which may affect water quality, which in turn will affect your fish. A build up of such gases is likely to inhibit fish from excreting similar toxic gases from their bodies. To prevent this we should look at ways of allowing gas exchange to take place.


Freezing ponds are an inevitability for the majority of us. Only a minority of Koi and garden pond owners are blessed with the foresight or means with which to heat their pond. Large scale pool heaters, which heat the water as it is pumped and recirculated through the filter system can be expensive to purchase, install and run. So how do we cope?

Short term measures

There is no need to treat a frozen pond like a failed pump or a leaking pond, because fortunately time is on your side. If you want to take action to ease your mind, thaw a small hole to allow some gas exchange by pouring some boiling water on to the ice. Ice should be thawed quietly , without banging or breaking the ice with physical force, because the pressure waves and noise can shock the Koi’s ears and sensory systems by stimulating their lateral line (their sixth sense which detects pressure). Any noise which you produce is an unnecessary shock. The Koi and other pond fish will be so lifeless at the bottom of the pond that they are likely to be oblivious to the presence of ice several feet above their heads. They will not be prepared for a sudden scare.

Long term measures

If the cold weather proves to be part of a longer freezing spell then a more reliable and labour-saving method to prevent a total ice cap from forming is required. Probably one of the most effective methods of providing a hole in the ice for gas-exchange is a small, inexpensive pond heater. Sold with a float, the pond heater (usually 100Watt) gives off sufficient heat to slowly melt a hole in the frozen surface. With the same running cost as a standard light bulb it really is worth the investment. Because it demands little energy the heater can be installed using a standard extension cable, protected with an earth leakage breaker. Like most effective solutions it is cheap, straight forward and simple to use. This is much more reliable than other innovations such as footballs and polystyrene space-like devices that have a tendency to become frozen in a solid ice-cap and will often go unnoticed until the surface thaws in the spring.


If you are victim to the frozen pond this year don’t panic:

Carefully pour boiling water onto the ice to create a

small hole.

Invest in a pond heater.

Avoid using footballs and polystyrene to conquer

the ice.