Winter
As temperatures drop, koi move into the deeper parts of the pond and become more reluctant to feed, and below temperatures of 6C(43F) cease feeding altogether. As the water continues to cool, the ko tend to lie huddled together
on the bottom of the pond with all fins except the tail fin clamped tightly against their bodies. This conserves some body heat, as each fin is supplied with a tiny network of blood vessels through which heat escapes if the fish lie with their fins outspread. One of the major areas of
cooling of a fish's body is through the gills, where the warm blood is cooled by the water passing over the gill filaments. You may have noticed that koi seem to breathe more slowly in winter - this is a method of conserving body heat. At temperatures between 2C and 6C (36-43F)
koi are described as being in a 'torpid state'. Their senses such as smell and taste are inoperative but they can still see and respond to some external stimulation, such as touch, or water movement below the surface.
Temperatures below 2C(36F) are critical to koi, as there is an increasing risk of ice crystals forming in the delicate gill membranes. Because water expands when it freezes, these crystals break the tiny cells in the gills and the koi die.
Winter filtration and aeration
Like many other materials, water expands when it warms and contracts as it cools. Because water is a liquid, warm
water is light and therefore rises. (If you feel your hot water storage tank, you will notice that it is hotter at the top than at the bottom.) Logically, you might expect, therefore, that freezing water (ice) should lie at the bottom of exposed water but, as we all know, this is not the case. In fact, a reversal of this principle occurs at temperatures just below 4C(39F), when colder water rises to the surface and warm water sinks. This is the salvation of
most underwater life. Water is a poor conductor of heat, so the cold water and ice form an insulating layer that retains
the heat of the warmer water below it. As we have seen in the section on pond and filter construction, the pond should be at least 1.5m(3ft) deep; depth will aid the temperature stability of the warmer water below the ice and the water will also absorb heat from the ground.
Keep all biological filters running throughout the winter. It is important that the water continues to ventilate so that oxygen can diffuse into the water and harmful carbon dioxide (the byproduct of respiration) does not become sealed in by total ice cover. If the pump is turned off, the fish will suffer and the plumbing will corrode or freeze if the pump is then left in the water. If a variable flow pump is fitted in the pond, turn it down to its minimum output. If your system enjoys the versatility of two or more pumps, then using one should be sufficient at this time of year in all but the very largest ponds. This will give you the opportunity to service any pumps not in use. (Most pumps are now sealed units and you need only clean them and check the strainers, cables and switches.)
You may have a submersible pump in the pond or a variable level gravity feed pipe, if so, you can vary the level from which the filter is fed. As the water temperature drops towards the critical 4C(39F), raise the pump or filter feed to 15-23cm(6-9in) below the surface. This will avoid disturbance and chilling of bottom water and allow the koi to rest, while preventing the pond from completely icing over.
You may decide to moderate the water return to the pond. Restrict the air flow on your venturi, bypass waterfalls and turn off fountains, since all these means of aeration are no longer necessary, as cold water is usually rich in oxygen, and, if left on, will excessively chill the water. If your water return is via a straight pipe from a filter above pond level, terminate it with a 'T' piece or elbow pipework to prevent disturbing the lower water.
It is not a good idea to use rubber balls, hammers or kettles of boiling water to prevent or break up ice formations. Although the koi are torpid, they can sense any water movement, and breaking ice sends violent shock waves through the water, which can severely stress, and even kill, koi. (It is a sensation probably similar to someone suddenly crashing a loud drum close to your ear while you are sleeping and waking you with a start!)
Heating outdoor ponds
A growing number of enthusiasts are installing heating systems in their outside pond to keep their koi active and feeding, and thus growing, throughout the year. Avoid placing heaters in the bottom of the pond, however, as they can set up convectional currents that lift the warmer water, which chills as it rises towards the surface or towards a layer of ice, thus returning cold water to the koi.(You should also remove any airstones from the bottom of the pond in winter for the same reasons.) Instead, place small heaters in the filter system, preferably in the settling chamber or transfer ports. Any heating unit should be thermostatically controlled and capable of maintaining the water temperature at a minimum of 10C(50F), even through the
severest of winters. Rapidly fluctuating temperatures are highly undesirable and temperatures sufficiently high to activate the fish but too low for safe feeding can have a debilitating effect. Furthermore, between 0C and l0C(32-50F) the koi's immune system does not operate, but bacteria in the water are still active. If your heater cannot maintain an adequate temperature throughout the winter, use it only to help stabilize autumn and spring temperatures.
Indoor ponds
An alternative to heating outdoor ponds that is favoured by some koi-keepers is the installation of pond systems in the
garage, greenhouse or conservatory, and some enthusiasts even have permanent fish houses. Such pond systems are
usually smaller and may be used at certain times of the year, for quarantine purposes or for raising baby koi, or they may be rested throughout the summer and used only during the colder months, to protect koi from the chill of winter.
Use mature water from the existing pond rather than tapwater to fill the inside pond. In this way, you avoid subjecting the koi to a change in temperature and water quality when you move them inside. In addition, you could move a small portable filter to the inside pond when you transfer the fish, having matured it with established pond water. Alternatively, by transferring some of the media from the active outdoor filter, you can overcome the ammonia/nitrite problems associated with new filters and pond systems.