There are two opinions with respect to heating koi ponds. Many koi keepers believe that koi should be given a typical winter period, while others believe that heating a pond is essential to maintain koi in the best of health throughout the year.
But won’t koi freeze in the ice?
Fortunately, for the benefit of all aquatic life in temperate regions, an unusual phenomenon occurs to water when it cools. Generally, the density of a material increases as it gets colder. This is true for water, but only down to 4oC. Between 4oC and freezing, water in fact gets less dense, and floats above the denser 4oC water, eventually forming a layer of ice as the air temperature reaches freezing point. Koi remain in the lower layers, avoiding being frozen. If this strange characteristic of water did not occur, then all fish and aquatic life would freeze each winter!
Pond heaters are unlike other pieces of pond equipment, such as pumps, filters, air pumps and possibly UV’s in that they are an optional extra. They do share a similarity however in that they are regularly the topic for hot debate and discussion. If your pond freezes in winter – you will need to know what your options are.
The argument against heaters
Koi, as colourful descendents from carp, have a physiology that can tolerate typical winter temperatures, and in fact, the winter period is theoretically quite a safe period for koi. The threat from disease during winter is minimal with the cooler water not only slowing down the metabolism of koi, but reducing the multiplication and infection rates of pathogenic bacteria and parasites.
As koi are poikilothermic (i.e. they take their body temperature from that of the surrounding environment), their metabolism has no option but to follow the trend set by the water temperature. Consequently, the metabolism, appetite and rates of excretion are all drastically reduced in winter, leading to less ammonia and solid waste needing to be removed by filtration, i.e. there is little or no risk of the pond water becoming toxic during this period. The colder water experienced by koi over winter also holds a greater capacity of dissolved oxygen (D.O.) preventing that from being a problem for fish.
But won’t koi freeze in the ice?
Fortunately, for the benefit of all aquatic life in temperate regions, an unusual phenomenon occurs to water when it cools. Generally, the density of a material increases as it gets colder. This is true for water, but only down to 4oC. Between 4oC and freezing, water in fact gets less dense, and floats above the denser 4oC water, eventually forming a layer of ice as the air temperature reaches freezing point. Koi remain in the lower layers, avoiding being frozen. If this strange characteristic of water did not occur, then all fish and aquatic life would freeze each winter!
The formation of ice across the entire surface should be avoided at all cost as it prevents the exchange of gases between the water and air, leading to a build up of CO2 and other noxious gases. A hole in the ice can be maintained by using a small 100W pool heater (or perhaps more aptly named ice-preventer).
Koi, particularly mature specimens, appear to benefit from a cold period as it re-sets their biological clock, preparing it to breed in the spring as the daylength and temperature begin to increase.
In conclusion, those who advocate a natural seasonal cycle for their koi believe that this is the natural and proper approach for over-wintering koi. But is ‘naturally’ really appropriate when considering such an unnaturally occurring fish?
The case for pond heaters
Systems for koi ponds provide the koi keeper with complete environmental control. They remove the unpredictable factors associated with UK winters, which are rarely experience a gradual decline in water temperature or increase towards spring. They also provide a means of taking the sting out of the extremist of winters (in both temperature and duration), removing the risk of koi becoming marooned under ice for months at a time.
Pool heaters also allow koi to be viewed all year round keeping them active, whether they like it or not, by maintaining a suitable temperature. This means that koi will continue to feed, and the pond maintenance will be required throughout the winter as though it were spring or summer.
Active koi throughout the winter are prone to infection just as they would be through the warmer months of a natural pond so vigilance would be required throughout the year.
The fact that koi will not spend weeks sitting motionless on the bottom of a pond will reduce the likelihood of them gaining sores from the abrasive pond bottom. They can become infected in spring as waters begin to warm up.
The period when koi keepers traditionally have their most difficult time with health problems is during the transition from winter to spring. As waters begin to warm up, pathogens are able to multiply at a more rapid rate than koi can defend themselves leading to an increase in the likelihood of disease. If a pond is heated over winter, then this risky period is removed from the koi keepers calendar.
If on balance, you opt to heat a koi pond, there are several ways by which it can be carried out. The two most practical sources of heat are by gas fired boiler or by electric heaters.
Electric Heaters
Although cleaner, more compact and easier to install than gas fired heaters, electric pond heaters are not as common as their gas-fired alternative.
Electricity is a relatively expensive means of heating water, making it quite a prohibitive exercise. They can be installed with a thermostat, and many units come with an integral control box. Rated from 3kw (for the very smallest pond) up to 18kw (suitable for approx 10,000 gallon, max). The most compact electric heaters that are easiest to install comprise of a simple L-shape design, where the pond water is pumped in one end of the L and out of the other, back into the pond or filter. Within the pipe is the heating element, the length of which is dictated by the capacity wattage of the heater.
These heaters are designed primarily for heating private swimming pools, and as a result, show little regard to any toxic metal elements that may be released into the water. Top of the range heaters comprise of stainless steel pipework and elements which are relatively inert and will not release toxic elements into the water.
The more powerful units are likely to benefit from their own dedicated pump as they are fitted with a flow switch that will cut the heater out if the water flow is not sufficient to dissipate the heat.
As the whole unit is not water proof, it must be sited externally to the pond, in a similar fashion to an external pump. Furthermore, a dedicated power supply would be required to safely take the load of the heater when it is switched on. A job for a qualified electrician.
Gas Powered
There are essentially two types of heater set up when considering a gas fired pond heater. Direct and indirect heating systems. Gas fired heaters are more economical and offer greater capacity than electric heaters. However, they are considerably larger, and require considerations such as a gas supply and flu.
Direct Systems:
These involve pond water being circulated to the boiler where an integral heat exchanger heats the pond water as it passes through. Care must be taken to ensure that the heat exchanger is not copper (as is often the case in these built-for-swimming pool units), as chronic metal toxicity problems will arise over time.
Indirect Systems:
These work in a similar way to the way water in a domestic hot water cylinder is heated. A closed circuit of water that does not come into contact with the pond water is circulated through the boiler. Once heated, the water is piped through a heat exchanger that is immersed in the pond or filter system. The heat exchanger is the equipment that must be inert (stainless steel) and in this case, it is of no consequence if the boiler has a copper heat exchanger, as this does not come into contact with the pond water.
The heating capacity of boilers and independent heat exchangers is rated at BTU/hr and it is prudent to fit a heat exchanger with a BTU/hr rating greater than that of the boiler. For example, to treat a 5,000 gallon pond, a domestic boiler of 40,000 BTU/hr and independent heat exchanger of 65,000 BTU/hr should suffice.
There are significant practical and financial implications when installing a heater for a koi pond. They are by far easier to install when the pond and filter system is being installed as retro-fitted heating systems will often lead to greater expense and problems of design and space when integrating them into an existing pond.
There are the running costs to account for as well, which will vary according to the extremity and duration of the winter and the efficiency of the heating system.
If in doubt, see if it is possible to visit and chat with a number of koi keepers who heat their ponds and ask for their opinion.
What will it cost?
When comparing electric and gas heaters, units delivering similar heating capacity should be compared. The cost of purchase (not including installation and running costs) for 18Kw or 55,000 BTU units are as follows:
Electric Heater: 600 + VAT
Gas Fired Adapted Swimming Pool Heater: 1100 + Vat
Gas Fired Domestic Boiler with stainless steel heat exchanger: 750 + VAT
Do the benefits outweigh the costs?
Many still believe that both koi and koi keeper benefit from a quiet period each winter and a time to reflect on the past year and plan for the next. It is certainly the cheaper alternative.
