Don’t Let Your Koi Know It’s Freezing Out

Heating your koi pond can bring some huge benefits for both you and your koi, and may not be as expensive as you think.

Take control of your life

If you live in an area that routinely freezes in the winter, adding a heater would allow you to control your pond, rather than allowing your pond to control you. Without a heater, each year you’re forced to go through the whole process of winterizing your pond to prevent freezing. Depending on how your pond is set up, this process of winterizing can range from a two hour project to two days. If it is not done properly it can become very expensive not only because of the loss of equipment and plumbing due to freezing, but also the possible loss of your koi.

Additionally, there is the anxiety factor to consider. When the water temperature drops below 40 degrees it becomes extremely hard on most koi. All winter you watch your friends, day in and day out, sitting lifeless on the bottom of the pond trying to get through the cold winter. Granted, they are cold blooded animals so their bodies can adjust (within reason) to cold water, but it can be mentally hard on us. You fondly remember how during the summer months, they would excitedly swim to greet you each and every time you approached the pond. With a heater on the pond, neither of you need to experience this heartache.

Serious health benefits for your koi

Not only does a heater make the pond a more bearable place for your koi during winter, it provides some real health benefits as well. Come spring, when the water temperature reaches approximately 52 degrees, any parasites and bacteria that may have been in the pond over the winter will start to become active. Unfortunately, the immune system of a koi doesn’t really become active until the water temperature reaches approximately 58 degrees. So for that time period when your water temperature is between 52 and 58 degrees, which can sometimes last several weeks, your koi are weak and somewhat defenseless. This time period is referred to as “aeromonas alley” by some because aeromonas, which is a common flesh eating bacteria that affects koi, is allowed to infect your koi without much resistance on the part of the koi.

Additionally, if one of your koi does becomes sick during the winter months, it can be a challenge to effectively treat it without the ability to warm up the koi. Warming the koi up serves to stimulate it’s immune system. If you have a small sick tank with a heater on it, you can easily warm up and hold a few fish. But, what do you do if your whole collection gets sick and your sick tank is not big enough to hold them all? With a heated pond, both you and your koi are able to avoid all these potential problems.

Heating options

Here is where you have a lot of control over your heating cost. By balancing your heating desires with how much it costs to heat your pond, you can settle on a heating program that fits the needs of both you and your koi. The following are some examples of how you might use your pond heater.

Example 1

You can just set the heater to maintain a minimum temperature all year long without ever touching it. This means that in the fall as the water temperature drops below that minimum temperature, the heater will automatically come on and prevent the pond temperature from dropping below that set temperature. As the outside temperature raises come spring time, the pond water temperature will rise above that set temperature, and the heater will simply shut off for the remainder of the year. If you choose to operate your heater this way, the temperature you select is going to be very important. If you set the pond temperature to 68 degrees, your koi will love you. They will stay extremely active and eat like pigs all winter long. If you set your pond temperature at 62 degrees however, your koi will still eat and be happy, but will not be quite as active. This lower temperature could save you at least 25% on your pond heating bill.

Example 2

You could set the heater to a minimum pond temperature of 50 degrees for most of the winter. This will allow the koi to go semi­-dormant, which means they will stop eating but will still swim around freely. They of course will not be pleased with you, but they can survive well at 50 degrees. This prevents the water from dropping into the stressful temperatures below 40 degrees, where the health of the koi becomes an issue.

When spring arrives, the water temperature will naturally rise to about 55 degrees. At that point, you can utilize the heater to gently raise the temperature to 65 degrees over a two day period. This will allow you and your koi to avoid “aeromonas alley”, the period where the parasites and bacteria become active while your koi’s immune system is not. This is probably the most economical way to use your heater while still providing a great benefit to your koi.

Example 3

You could use your heater as an emergency tool. In this scenario you may go through the entire winter without using the heater once. However if you did experience a health issue with one or all of your koi, you have the ability to gently warm the pond to a temperature sufficient to treat the problem. Additionally, if your koi were to ever come in contact with the KHV virus, you have the option to raise the pond temperature to 86 degrees which to date is the only hope of stopping the virus from quickly killing all your koi.

Some important notes to keep in mind. You never want to raise the pond water temperature more than 5 degrees per day or lower it more than 3 degrees per day. Sick koi respond to treatment very well at water temperatures between 65 and 78 degrees. It is very important to remember that as you increase the water temperature to stimulate your koi’s immune system, you are also stimulating the reproduction of any parasites or bacteria in the pond. Therefore, any scheduled treatment should be started as soon as possible. You may even want to consider starting the treatment a day or two before you begin to raise the temperature.

Selecting a heater

There are some important things to look for in selecting a heater:


The efficiency of a heater can be a make or break issue for heating your pond. If the heater you select is so inefficient that you can’t afford to operate it, you have wasted both your time and money. In essence, make sure the efficiency rating is as high as possible. Also, consider how far away from the pond the heater will be, and whether or not it uses a heat exchanger. Heat exchangers, though efficient, still lose some efficiency in the exchange process compared to directly heating the pond water.


The heater needs to be extremely dependable. An important aspect of heating a koi pond is to reduce the stress on your koi. If in the middle of the winter the heater stops heating for any reason, your water temperature can drop rapidly. This can be extremely stressful for koi, since a 20 degree drop in temperature over a few days can be life threatening.

Gas heaters seem to have an advantage over electric heaters, not only because gas in general is cheaper than electricity, but also because gas supply interruption is seldom a problem. If during the winter you experience an electrical outage, both a gas and an electric heater will stop heating. The difference is that a gas heater typically requires less than .5 amps to keep the electronics working, whereas an electric heater may require 40 amps or more to operate. With a gas heater, all you need is a small gas generator to keep it running if an outage were to occur.

No copper

Copper is toxic to koi! At high levels, copper can kill koi. However even at low levels it has been proven to greatly lower their immune system, making them more susceptible to getting sick. Many swimming pool heaters use copper for their heat exchangers because of it’s ability to transfer heat exceptionally well. When the copper is heated to a high temperature though, it begins to shed copper into the water at a much faster rate than a standard copper water line.

The ability to maintain an accurate temperature

Look for a heater that will maintain the temperature within a couple of degrees. Some heaters are only capable of maintaining the water temperature within a 5 degrees range. What happens then is that the heater will bring the pond water temperature up to say 62 degrees, and then shut off. It will not come back on until the temperature drops below 57 degrees. That can be very stressful to koi on a continuous basis. Ideally, you do not want to allow the water temperature to fluctuate more than 2 degrees per day.

If you are one of those people who really enjoys the whole koi experience, heating your pond may be just the right pond improvement for you. Heating your pond will increase the amount of time you will have to enjoy your koi and greatly reduce the time you spend worrying about them. The confidence you get by knowing you have so much more control over your pond, plus the way a heater can simplify caring for your pond over the winter can have a profound effect on your whole koi keeping experience.