Do you need to heat Koi Ponds?

Is heating Koi Ponds in the winter important?

Koi are not cold-water fish and cannot thrive in temperatures near freezing. They share traits with Carp, from which they were bred, including the ability to adapt their behaviour to a wide range of temperatures.  Koi are more active during the summer when their appetites increase to match their activity. As winter approaches and temperatures drop, Koi become less active and lose their appetite, especially in unheated ponds. When the temperature reaches about 10°C, Koi have no appetite and will spend most of their time at the bottom of the pond. At temperatures of 4°C and below, they huddle together at the bottom of the pond to conserve energy until the weather warms up. Koi cannot generate internal heat to stay warm, but they can generate heat by shivering, which rapidly contracts and relaxes their muscles.

The argument is that Carp evolved in lakes, no matter how severe the winter. So, they must be okay in colder temperatures, right? Yes, they managed to survive because, in natural lakes, the water temperature near the bottom does not fall massively below 4°C. However, the Koi could not do anything but wait for warmer temperatures.

When two different water temperatures come into contact, temperature stratification occurs. The difference in temperature causes the colder, heavier water to settle at the bottom, allowing the warmer water to float above. This process prevents water in lakes from dropping below 4°C in lakes, but it cannot occur in a pond.

As the sun shines more on the water during spring and summer, the water gets warmer. In the transition from summer to winter, the water at the surface remains warmer than the water below because less sunlight reaches the lower levels.  Additionally, the water from the filter pump and air stones keeps mixing the colder water with the slightly warmer water. As a result, the entire pond can drop below 4°C, putting the fish at risk from temperatures they are not naturally equipped to handle.

Not all Koi will die as a result, but a large number were reported to have died in unheated ponds last winter (Dec 2022 – Feb 2023). Covering an unheated pond is an effective way to prevent this from happening. It’s also a good idea to have a heater set at 4°C for ponds where covering alone is insufficient, especially for individual installations and locations.

Even if we disregard the ethical concerns about exposing Koi to these conditions, the financial cost of replacing Koi can be higher than the cost of the minimal amount of heat needed to prevent a properly covered pond from dropping below the standard temperature.

How do heated ponds affect the Koi immune system?

A study by Morvan, Troutand and Deschaux found that while the nonspecific immune system may work harder to compensate at lower temperatures, the immune system is very much impaired as a whole when temperatures are below 12°C.

Heating ponds in spring and autumn

Bacteria such as Aeromonas can infect Koi until the temperature falls below 4.4°C, at which point it becomes inactive. As temperatures increase, the fish become increasingly active, reaching their peak activity at 15.6°C. Below 12C, the Koi immune system is less effective, allowing Aeromonas to affect them until the temperature falls below 4.4°C. This temperature range is known as Aeromonas Alley. If the pond stays in this temperature range for extended periods, these bacteria will have the advantage of attacking when the Koi defence systems are weak.

The Aeromonas Alley effect illustrates the importance of maintaining some level of heating from autumn to spring. For example, setting a heater thermostat to around 13°C can help protect Koi from bacteria. The heater won’t need to operate during the summer, but as the temperature drops in the winter, it will keep the water from falling below 13°C, ensuring this temperature is maintained throughout the winter. However, some believe that modern Koi, due to breeding, have strayed too far from their ancestors and should be heated throughout the entire winter, possible at a higher tempertaure. Others argue that Koi require a “winter” period similar to what their ancestors experienced in natural lakes during their evolution.

People who want to keep their ponds heated year-round should maintain the temperature at 13°C or above to prevent their Koi from experiencing Aeromonas Alley. Those who want their Koi to experience a winter period can gradually lower the temperature from 13°C to 4C by adjusting the thermostat in stages. This gradual decrease in temperature will prevent it from getting too low. In the spring, as the temperature starts to rise, the thermostat can be adjusted gradually back to 13°C to maintain that temperature until the warmer weather naturally heats the water above that point. If the pond is covered during the colder months, this approach will enable the Koi keeper to give the Koi a winter period without incurring a large heating bill.

Heated ponds and dissolved oxygen levels

Many species have “determinate” growth, meaning they grow to a certain size and then stop. However, fish have “indeterminate” growth, so there is no limit to their size. While Koi may grow more slowly as they get larger, as long as they have a proper diet and temperature range, they will continue to grow. The availability f oxygen is a limiting factor that affects the growth rate, and warmer water could harm this rate.

Temperature °C	0	5	10	15	20	25	30
Oxygen mg/L	14.6	12.8	11.3	10.1	9.1	8.2	7.5

The following table displays the oxygen saturation levels in fresh water, indicating the maximum amount of dissolved oxygen that pond water can hold. As water temperature increases, the capacity to hold oxygen decreases. Regardless of how many extra air stones are added, the dissolved oxygen level cannot be increased above the values in Figure 1.

As a Koi grows and doubles its length, its width and height will also double. This results in an eight-fold increase in body mass. All the extra body mass requires more oxygen to function. Although the size of a Koi’s gills will increase as it doubles in length, their size does not increase by eight times. This is a natural effect and does not mean that fish suffer from oxygen deprivation as they grow.

As fish grow larger, the gills are less effective at providingenough oxygen to its body compared to when it was smaller. This is why small fish grow quickly but their growth rate slows down as they get bigger. If you heat a Koi pond to promote better growth, it’s important to ensure proper aeration to make up for the reduced ability of water to hold dissolved oxygen, especially if the water is warm, which lowers the dissolved oxygen level.

In order for Koi to effectively digest and metabolise food, it’s important to provide them with a sufficient oxygen supply. If a Koi pond is heated to promote growth, it is counterproductive if the water’s dissolved oxygen level is not sufficient to allow the fish to obtain the maximum energy from their food. Therefore, it’s essential to ensure the highest level of aeration is achieved when heating a pond to promote good growth.

Heated ponds and growth

The Q10 Temperature Coefficient Theory in science relates to chemical and biochemical processes controlling fish metabolism. It states that these processes will occur twice as quickly if the temperature is raised by 10°C. However, factors like the decrease in availaible oxygen can affect koi metabolism, so an increase in water temperature by 10°C may not exactly double its activity. Nevertheless, raising the water temperature from 15°C to 25°C will lead to significantly increased koi metabolism and activity.

Opinions of the ideal water temperature for maximum growth may vary, but 25°C is generally the main temperature chosen when budget is not a constraint. The perfect temperature range for carp is often considered to be between 20°C and 30°C, making 25°C the preferred temperature as it falls in the middle of this range. Slightly increasing the water temperature above 25C can boost a koi’s metabolic rate and its growth potential, but there is a diminishing return in growth per gram of food as the temperature rises.

Increasing the temperature by 10°C will roughley double the koi’s metabolic rate, potentially allowing it to grow twice as quickly at 25°C compared to 15°C. However, increasing the temperature further by another 5°C to 30°C will not increase the potential growth rate by an additional 50%. Besides being near the maximum temperature a koi can tolerate, the diminishing capacity for water to hold dissolved oxygen is an important factor in the relationship between food and growth.

Resting koi require 6.0 mg/L of dissolved oxygen, while actively swimming koi require closer to 7.0 mg/L in order to fully power their muscles. Higher amounts of dissolved oxygen, above this level, can be used to metabolise food and build body tissue. Figure 1 shows that at 30°C, the maximum amount of oxygen that water can hold is only 7.5mg/L under ideal conditions. Although highly aerated pond water can approach the values in the table, in practice, as long as there are fish and other life in the pond that are using oxygen as it is added, the actual values in the water will always be slightly lower.

When fish digest food and convert it into body mass, they need a lot of oxygen. So, when the temperature rises above 25C, there might not be enough oxygen available for both swimming and digestion. This limits the fish’s growth rate. In higher temperatures, fish may eat less due to limitations, and some food they eat may not be fully digested and will be excreted as waste.

The growth rate of Koi depends on how quickly they can eat and metabolise food. Simply doubling the feeding regime doesn’t guaantee that the growth rate will also double, as the fish need to be able to metabolise the extra food to build a bigger body. However, if the water temperature increases from 15°C to 25°C the metabolic rate will approximately double, increasing the capacity for the growth rate to double as well. It’s important to note that if the metabolic rate doubles but the feed rate and quality of the food don’t keep up with the increased metabolism and appetite, the growth will suffer.  This is because the koi will be more active and will expend more energy, similar to how we might nee dextra exercise for weight loss or control.

Keeping energy costs to a minimum

If you heat a 3,000-gallon pond only in spring and autumn to around 13°C and in winter to prevent the temperature from falling below 4°C, the cost can be less than £1 per day, even with an electric heater, but only for the days that the heater is used. Those who heat their ponds all year can end up paying much more, depending on the water temperature, the amount of water that is changed, and the weather. Some koi keepers change as much as 10% of the water per day, so it’s not surprising to hear of bills of £1,000 or more per year.

An effective way to minimise the cost of heating a pond during winter is to cover it and prevent heat loss due to wind chill. Even when a pond is uncovered in the summer, it’s important to insulate the heat inside.  The best time for pond keepers to reduce heat loss through the walls and floor is during pond construction.

After beginning pond construction, there may be a rush to complete it quickly for filling and stocking. However, once the shell is built, it’s important to take the time to ensure that future operating costs are minimised, Investing in thermal insulation boards is a wise decision that will bring significant long-term benefits.

Extruded polyethene insulation is created by blending polyethene pellets with a substance that triggers a foaming reaction. As this reaction occurs, it is extruded into flat sheets of various thicknesses depending on its intended use. Afterwards, it solidifies into a closed-cell plastic foam that is resistant to moisture and possesses a degree of flexibility. This type of insulation offers good compressive strength, which means it can withstand the water pressure when the pond is filled without getting crushed.

Polyurethane foam insulating sheets are created by blowing a non-CFC gas into urethane resin to produce foam. The foam is then coated with aluminium foil, allowing it to be passed through rollers to control its thickness as it hardens into flat insulating sheets.

When the pond walls and floor are flat and reasonably smooth, you can use either type of board and fix it in place with suitable mastics or adhesives. The sheets mustn’t span any uneven areas or “voids” in the walls or floors. If a liner is fitted, it will be flexible enough to stretch into place, if the sheets are pushed outward into these voids by water pressure when the pond is filled.  While fibreglass finishes are strong, they are not designed to flex when cured. So, it’s important to properly support the sheets to avoid unnecessary stress and potential cracking or splitting of the finish.

Where the walls are curved or the floor is uneven or irregularly shaped, large flat sheets cannot be attached smoothly to surfaces. Therefore, one possible solution may be to cut a full-size board into smaller pieces that can be fixed individually.  When the shape of the walls or floor is too curved for this method to be practical, another option is to have polyurethane foam sprayed directly onto the pond surfaces by a specialist company. While this isn’t cheap, the cost can be offset by the savings made on the cost of rendering the walls and screeding the floor.  The foam can be sprayed onto any surface, no matter how uneven, so there’s no need to render a wall beforehand. Also, there’s no need to screed a concrete floor to give the pond a smooth bottom; the foam can be sprayed directly onto the rough finished concrete surface and then easily sanded to remove any unevenness due to uneven application.

Heating ponds by electricity

The cost of heating a pond throughout the year can be challenging to calculate because it depends greatly on the pond’s size, whether it’s insulated to prevent heat loss and the temperature difference between the water and the surrounding environment. Additionally, the frequency of water changes will significantly affect heating costs; the more water changes, the greater the cost of heating new water.

While not the most cost-effective heating option, the in-line electric heater is the cheapest and potentially the easiest to install. Good quality heaters are available for less than £300 if you shop around.  These heaters are compact and can be easily incorporated into existing setups by cutting into a straight length of pipework. However, it’s important to note that not all pipework will be suitable for installation. The process is straightforward, but some fundamental rules should be followed during installation.

The heating elements in an electric heater should always be fully submerged in water while in use. If the water flow doesn’t cover it completely, its elements can burn out quickly. High-quality heaters typically have a flow switch to turn them off if the water flow stops. However, these heaters and flow switches were originally designed for clean, sterile water, such as swimming pools right after a sand filter.

When pond heaters are used to heat ponds, they are in a completely different environment. Pond water cannot be sterilised by disinfectants such as chlorine, and the water is not as clean as that in an average swimming pool. It is possible that the heaters can become clogged with pond debris, odd strands of blanket weed, or the biofilm that can build up on them.  Any of these can cause the heaters to be unable to switch off if the water flow through the heater stops.

It’s generally recommended to turn off the heater before shutting down the filter pump or performing any tasks, that could disrupt the water flow. However, haste or inattention can sometimes cause us to forget this important step. Additionally, it is possible for the pump to lose its prime or for the water flow to stop due to a blockage.

The flow switch is designed to protect the heater elements in certain situations. A clean flow switch is very reliable, but if it becomes jammed and cannot be switched off, it won’t be able to protect the heater. In such a scenario, a simple mistake like backwashing a filter or flushing it to waste without turning off the electric heater first could be quite costly.

To avoid potential issues, do not install in-line electric heaters in any section of the pipework that could drain if the filter pump is turned off, if the water flow is interrupted, or if a valve is accidentally left in the wrong position.

Electric heaters should only be installed in pipework that is always filled with water. This ensures that even if the water flow stops, the heating elements will remain submerged.  Installing an electric heater in this manner has the advantage that if the water flow is interrupted while the heater is on, the remaining water will start to heat up. The built-in temperature sensor will detect this and switch off the heater, even if the flow switch is stuck in the on position.  It’s important to note that all installations are unique, but as a general rule, any pipework that is higher than the water level in the pond has the potential to drain or collect air when the pump is turned off, so it’s best to avoid these areas when considering where to install in-line electric heaters.

Pond heaters are measured in kilowatts (kW), and a good rule of thumb is to use 1kW per thousand gallons, which should be sufficient for most conditions. Remember that bigger heaters are not necessarily better.  Assuming there are no significant heat losses, a 1kW heater can raise the temperature of 1,000 gallons of water by 1°C in a little over 5 hours, which is a reasonable rate of temperature change for koi. However, if left on, for a whole day in a cold pond, this would result in a rise of 4.5°C in 24 hours, which is at the upper limit of what koi should experience. On the other hand, a 3kW heater would raise the temperature of 3,000 gallons at the same rate. If used in a smaller pond, such as 1,500 gallons, it would heat the water at a rate that is too fast.

Direct heating gas boilers
Swimming pool boilers that use direct heating typically cost around £2,000. Water is pumped into the boiler and heated directly by the gas burner.  These boilers are specifically designed to efficiently heat swimming pools to approximately 30,000 gallons to temperatures around 30°C.  They can be controlled by thermostats to lower temperatures, but it’s important to note that they are designed to heat large volumes of water very quickly. Therefore, if used on small koi ponds, the rate of temperature change will be very fast.

Thermostats are not installed on the output of these boilers because that would cause them to keep switching off as soon as they started heating the water. The control thermostats have to be installed where they can sense the water flowing into them, which means that they cannot sense the new temperature of the water until the heated output water has circulated throughout the pond.  With a two-hour pond turn-over rate, this means that when these boilers switch on, they won’t sense a temperature rise and switch off until the heated water is pumped back into them again two hours later. Since the rate of temperature change will be fast and the boiler will have been heating for at least two hours, the resulting temperature fluctuations on a small pond can be extreme.  For this reason, I wouldn’t recommend using direct-heating swimming pool boilers on a small koi pond. 

Domestic gas boilers
Gas boilers cannot directly heat pond water. Instead, they use a heat exchanger to heat a coil of narrow bore pipe inside a water jacket to a temperature between 70°C and 90°C. The pond water is pumped through the water jacket, where it flows over the heated pipe and picks up the heat. The heated water from the boiler is sealed inside the internal coiled pipe and does not mix with the pond water. Heat exchangers come in different sizes based on the volume of water they are intended to heat and the temperature they are expected to achieve.

The range for a standard-sized heat exchanger is from 60,000 BTU for less than £200 to over 400,000 BTU costing over £600.  These sizes indicate the heat transfer rate from the boiler to the water to be heated.  Simiar to electric heaters, there’s no benefit in selecting a larger BTU rating than necessary because doubling the rating will (double the rate of temperature change when they are heating the pond. A 60,000 BTU heat exchanger, when supplied from a boiler with enough spare capacity to provide the correct temperature primary flow (the water circulating between the boiler and the heat exchanger), is more than enough for the average-sized pond.

To determine the suitability of each installation, it’s important to consider its specific circumstances. However, in most cases, even with a minimum-sized 60,000 BTU heat exchanger, the rate of temperature change will likely be too high. A simple solution for plumbing in heat exchangers is to use two tees to create a bypass loop across the pond water input and output of the heat exchanger. You can control the water flow by installing a gate valve in the loop. When the valve is closed, all water must pass through the heat exchanger, but much of the water will bypass it when the valve is fully open.

When the valve is closed, the heat supplied to the pond will be at its maximum, causing the temperature to rise rapidly.  When the valve is fully open, the rate of temperature rise will be much slower. When the boiler is initially heating the pond, with the valve set halfway open, it’s important to closely monitor the rate of temperature rise and adjust the valve to acheive a suitable rate. This adjustment can be fine-tuned over a few weeks, after which it may be a good idea to remove the handle/wheel to prevent it from accidentally being altered from this optimum setting.

You can install a domestic gas boiler and heat exchanger specifically for the pond, or if the pond is close enough to the house and the central heating boiler is conveniently located, you can run an extra feed to the pond heat exchanger. If you need to run the primary feed from the boiler to a heat exchanger near the pond, insulate the pipes well to prevent heat loss.  This is important because heat loss would not only increase the heating bill for the house unnecessarily but also render the heat exchanger won’t be effective if the primary feed isn’t in the correct temperature range. (Checked up to here)

A slightly more efficient way to run a heat exchanger from an existing house boiler is to fit the heat exchanger in the house near the boiler and run the pond water to the house.  This will generally not be convenient in practise because of the size of the pipes that will have to be run and the extra trouble involved in insulating them but, if it is possible, there will be an energy saving. This is because, assuming that the insulation thickness is the same, pipes carrying water at normal pond temperatures lose far less heat to their surroundings than pipes carrying water with a temperature above 70°C. There will also be the advantage that the heating controls for the heat exchanger can be easily linked into the central heating controls with the central heating and the heat exchanger controlled by separate zone valves so that the central heating and the pond heating can operate independently of each other.

Heat pumps

There are a few installation constraints on fitting a heat pump. A heat pump needs to continuously draw in large volumes of air so it should be sited where there are no restrictions to the airflow. The exhaust air will be cold so it shouldn’t blow into an area where you might like to sit. The continuous cold air from a heat pump will feel like winter to shrubs or garden hedges, don’t let the exhaust blow directly onto them or they might die back. Don’t fit a heat pump in a shed or greenhouse thinking that it is warmer inside them so the extra heat will be transferred to the pond; the heat pump will quickly cool the air and then be unable to draw in warmer air from outside.

Originally designed to heat swimming pools, these are a very efficient way to heat ponds in summer.  They work by pumping huge volumes of air through two internal heat exchangers which extract heat from the air flow. This heat is passed to a third heat exchanger which similarly heats water to those described above. They don’t need sunshine and will operate on cloudy or even rainy days as long as the air temperature is warm.

A small heat pump can cost less than £500 and will use typically in the region of 1kW of electricity, whilst it is running. The output is rated in terms of COP (coefficient of performance) which is a measure of how many times more efficient it is than an electric heater of the same rating.  For example, a heat pump with a COP of 6 will produce an output equivalent to a 6kW electric heater for each 1kW of electricity it uses.  Small ones typically have a COP rating of between 4 and 6 when the air is warm.

The COP is dependent on air temperature and can easily fall to 3 or less in cold weather. This would mean that they only give the equivalent of 3kW out for every 1kW of electricity they use which is still a good saving but this may not be sufficient to heat a pond during winter when heat losses are at their greatest and air temperature is at its lowest. Anyone intending to purchase a heat pump for winter use is best advised to question the supplier about its performance in winter to ensure that it will maintain the desired pond temperature without temperature fluctuations caused by there being insufficient heat output on the coldest days.

Unarmed verbal combat
On this magazine’s forum, a couple of years ago, I entered into a discussion with a manufacturer/supplier of a popular heat pump.  He was very convincing and I felt better about the performance of his product in cold weather after our discussion but there was one guarantee I couldn’t get him to make.  That guarantee was that his heat pump would always maintain a pond temperature without fluctuations in severe winters.  He wisely said that to ensure a constant temperature in the severest of winters, it would be best if there was a backup form of heat such as an electric heater that could take over if the weather didn’t allow his heat pump to produce enough heat.  I agree.  Heat pumps are designed to produce heat even when the air temperature is well below freezing so they will probably be able to maintain a stable temperature throughout most winters. But if the health and welfare of your koi are at stake, it would be a wise investment, to also install an electric heater. This would probably not be needed in most winters but would be there as a backup, just in case.

Solar heating

One way to monitor and record water temperature is with a pen recorder that will draw a line representing the temperature on a continuous roll of paper.  Variations will be shown by the “waviness” of the line but this equipment is expensive. An inexpensive way to monitor pond temperature variations is by purchasing the type of digital thermometer that, not only displays the current temperature but also stores maximum and minimum temperatures that were reached in any particular period since it was last reset. These can be obtained online for under £20.

If a pond is to be heated it shouldn’t be done half-heartedly. Solar heating is a good example of where heating pond water can be detrimental rather than a benefit. Solar heating can be a valuable addition to a pond that is already heated by some other means since whatever heat can be gained from the sun will reduce the running costs of the main source of heat.  However, the problem with solar heat in the United Kingdom is that the available amount varies greatly from week to week and even from day to day.

It is tempting to feel the warmth of the water in a hose that has been left in the sun and imagine ways in which this heat could be put into the pond.  The simplest way to do this is to have a coil or zig-zag hose in the sun, then pump pond water through it and allow the heated water to flow back into the pond.

In summer, on a sunny day, a great deal of heat can be gained this way and, if this is the sole form of heat, the pond will be much warmer on those days. But the sun doesn’t always shine. It is quite common for there to be good sunshine one day and very little the next so the pond will warm up on sunny days and cool back down again on cloudy days. Unless there is another, more reliable, heat source to take over when there is no sun, the constant varying of pond temperature will alter the koi’s metabolism very quickly and this will cause it unnecessary stress.

Using solar heat in the winter without a reliable alternative could be even worse. When the pond is cold, a couple of days of reasonable sunshine in the winter could raise the temperature of the pond, but the several days between sunny periods will allow it to go cold again.  The koi will be in their “winter mode” one day, then the water temperature will warm and they will think spring has arrived early only to find that they are back in winter temperatures straight after. Repeated wild fluctuations in pond temperature during winter could be extremely detrimental to them.