Frozen Fish Ponds. What if Koi Pond Freezes in Winter?

The following is the process whereby water in a frozen fish pond actually freezes:

The air temperature drops below pond's water freezing point (zero Degrees Centigrade). A thin film of ice forms over the surface of the koi fish pond.

The term frozen fish ponds implies a solid lump of frozen fish pond. Only in the very worst Siberian type winters and shallow ponds does a pond freeze solid. Frozen koi or gold fish ponds are generally referred to when the surface of the fish pond is frozen. This is the type of frozen fish pond we will discuss on this page... better described as a fish pond frozen over or a garden fish pond with a layer of ice.

As long as the air temperature is below freezing point of pond water (zero degrees Centigrade... 32 degrees Fahrenheit the layer of ice on the fish pond surface will gradually get thicker. As the ice layer gets thicker on the fishpond it becomes a kind of heat insulator and the rate of temperature drop in the water below fish pond ice surface falls. And this is what is really clever and allows us all not just koi or other pond fish to survive.

Water Has Its Maximum Density at 4 Degrees Centigrade

Water has an important physical property and is unique in this behaviour. It is this property that allows aquatic life including pond fish to survive when severe freezing in winter takes place. Let me explain what happens in a fish pond as water freezes, trapping the fish below a solid surface preventing any air or oxygen transfer from the air to the water.

The density of liquids increase as temperature of the liquid drops. So you might expect the cold surface pond water of zero degrees Centigrade would drop to the bottom of the fish pond. If this was the case fish pond water would then freeze from the bottom up.

Water has its maximum density not at zero degrees but at 4 degrees Centigrade. This means water below 4 degrees will rise to the surface and insulate the lower levels of fish pond water. Ice formation on the koi pond surface also insulates the fishpond water to the extent that it is  extremely rare that a fish pond of any kind and any lake for that matter will freeze solid.

If you measured the bottom layer of pond water in a frozen fish pond you would get a measurement of 4 degrees. At this temperature fish are very sluggish and therefore consume little oxygen. This is how koi or other pond fish survive a freezing winter air temperature and frozen water surfaces.

In summary koi and all other pond fish subjected to very cold winter environments know what's best for them... they sink to the bottom of the pond and become completely motionless.

This phenomenon of pond water having its maximum density at 4 degrees (water above and below 4 degrees C is of a lower density) is one of nature's secrets to survival. Amazing isn't it?

In summary a frozen pond will not result in the koi freezing to death even in very prolonged and cold weather (unless pond is very shallow).

By the way this is a very good reason why most koi pond experts suggest that a koi fish pond should have a minimum depth of about 4 feet.

During prolonged winter freezing conditions undesirable noxious gases remain dissolved in the water because they cannot escape through the fish pond surface as normal. Remember also that the processes of water purification (ammonia and nitrite removal) in the pond biofilter also slow down

It is therefore a good idea if prolonged freezing conditions are expected or encountered that a hole be maintained in the pond surface.

How to Stop a Fishpond or Koi Pond Freezing Completely...

There are simple low cost pond devices that allow a hole in the koi pond water ice surface to be maintained to allow these noxious gases to escape from pond water.

Not all of these devices use electricity. This is the Velda floating ice preventer ...

For those koi keepers who can afford it (and believe it or not many can) the ideal solution is to use an inline gas or oil fired heater to maintain the koi pond temperature at about 18 degrees C.

If you can't afford one of these (see links and images above) then you can always resort to Peter May's solution to creating a hole in the surface fishpond ice.

Peter advocates the use of a saucepan containing boiling water. The sauce pan is placed at a convenient point and the heat melts a hole in the fish pond surface ice so that pond gases can escape.

It's a good idea to buy a low cost floating thermometer to monitor koi or fish pond water temperatures. You can even monitor your pond temperature from your armchair using this clever Velda pond temperature device

And, by the way, if you don't want to get in the pond and clear out the muck then consider a pond vacuum cleaner or pondvac for short. The picture is of the hozelock pond vacuum model but there are a few different kinds you can choose from... from very cheap to the very expensive OASE Pondvac models are the best by far but also the most expensive

Can Koi Freeze?

I am often asked the question can koi fish freeze and for that matter can goldfish freeze? Yes if you have extreme and very prolonged temperatures below water freezing point. This is highly unlikely to happen unless the fishpond is very shallow.