The Optimal Koi Fish Temperature Range (Ultimate Guide)

Koi fish are quite hardy, but they should still be kept as comfortable as possible by giving them amply oxygenated, clean water, the best food you can afford, and lots of love and attention.

Having clean water, however, is not enough, as the water should also be the right temperature for koi to thrive at.

When it comes to a koi pond’s temperature, it’s important not to let the water get too cold or too hot. Koi fish prefer a water temperature of 65 — 75 °F (18 — 24 degrees Celsius) even though they can survive in water temperatures ranging from 35 °F to 85 °F.

Because even slight temperature changes can have an impact on your koi, you should always have a thermometer in the pond water. This will make it a lot easier to keep an eye on water temperatures throughout the day and season.

However, when you do need to cool the water down or heat it slightly (with the changing of the seasons, for example), you should be careful not to let the temperature change too quickly as this is also bad for your koi and could send them into shock.

Next we’ll be looking at the correct and safe way to either heat up the koi pond’s water or to cool it down.

What To Do If Your Koi Pond’s Water Is Too Hot

Even though koi can survive for a time in water up to 85 F, this is already too warm for many fish, not to mention many aquatic plants. As water’s temperature rises, it contains less dissolved oxygen, while ammonia levels may rise and become toxic to the koi and other pond life.

If you find that your koi pond’s water is too hot, you need to jump in and make some changes. It’s important to know that water in a pond doesn’t heat up as fast as, say, water in a kettle.

It takes quite a while for the water temperature to rise through natural means, so you have time to step in and make the necessary changes.

How to lower a koi pond’s temperature in an emergency

If your pond’s temperature changes by more than 5 degrees or you find that it is higher than 75 F, you’ll definitely have stressed koi and need to do something to cool the water. To do this, you can:

• Create shade over your pond immediately, if you can.
• Change about 20% of the total water in your pond. Do this gradually to lower the temperature gradually as well and not cause shock to your koi.
• If you do change this amount of water, you’ll need to use a dechlorinator, as most cities treat their municipal water with chlorine or chloramines (chlorine and ammonia) as disinfectants. Be sure to use a treatment product that will take care of chlorine and chloramines.
• Then you need to boost the circulation of the pond’s water by using a 3,000 gallon per hour pump. This pump will keep the (now cooler) water cool and flowing.
• You can also add an air pump. This pump will create enough turbulence to ensure that more oxygen is dissolved in the pond water.

Remember to keep an eye on the water temperature while you take these steps to ensure that the water doesn’t cool too quickly, but changes only gradually, giving your koi a chance to adjust.

Once you reach the desired temperature of between 65 – 75 °F, you can stop taking steps to cool it down and focus on keeping the water temperature stable.

How To Prepare Your Koi Pond For Summer

There are steps that you can take to ensure that your koi remain happy during summer by keeping their pond’s water at the correct temperature.

By making some permanent improvements to your pond and its temperature, you will also make it easier to care for because it won’t overheat easily – or at all.

Get the circulation in the pond going by using a pump, waterfall, or stream – or a combination, depending on the size of the pond. This will ensure that the pond’s water circulates which keeps the temperature of the water down.

Shade is also an important part of keeping your pond’s water cool and shading your pond is easier than you may think. While a decorative bridge can bring a bit of relief from the heat, it’s also important to try elements like screens, shrubs, trees, and aquatic plants like water lilies.

Your pond should also have a space that is deeper than two feet. This is because the deeper water is cooler and can then be used as a refuge by your koi during the hottest part of the day. It will also help with cooling down the entire pond as the pump circulates the pond water.

To ensure that the water is as oxygen-saturated as possible, place air stones in the pond, waterfall area and even with your filters.

Remember that the plants also use oxygen during the evening, so you need to keep oxygen levels high to ensure that there is enough oxygen for everyone in the pond.

Feed your koi in the morning and late evening when the temperatures are lower.

You can also try adding a misting product to lower the pond’s temperature by lowering the ambient temperature above and around the pond.

Some fountains have a “misting” setting which you can use, but you can also get a fixture for your garden hose that will spray water in a fine mist. In this way, the immediate area can be cooled by up to twenty degrees (Fahrenheit)!

Not only do they use minimal water, but they are also very cost-effective.

If you find that your pond still keeps on overheating even after trying all these things, you will need to look at moving the entire pond to a cooler spot in the garden.

What To Do If Your Koi Pond’s Water Is Too Cold

If you find that your koi pond’s water is too cold or the temperature is changing by more than about five degrees during the night, you will need to step in and make sure that the water heat up again to a temperature where your koi will be comfortable.

• If the water temperature has dropped below 50 °F, do not feed your koi as they won’t be able to digest the food.

If it’s fall or winter, follow the steps below to increase the temperature of the water during the season. However, if it’s still summer and you find that the water’s temperature is too low, you will need to figure out why this is.

If you live in a very cold climate, it’s important to note that your koi — because they are also cold-blooded — will find it difficult to survive if even the summer temperatures don’t give the water enough time to heat to between 65 F and 75 F.

Your koi will then most likely have to be kept inside all the time in an aquarium to ensure their health and wellbeing.

However, sometimes it can be that you have just implemented too many steps to lower the water temperature during the summer. If this is the case, you can simply take away some of the elements that lower the water temperature like a mister.

You can trim trees and shrubs back, and thin out some of the water lily leaves that are covering the pond’s surface. This will allow more sunlight to get to the pond, which will naturally heat the water.

You can also use a plastic sheet to cover the pond (weighing it down around the edge with heavy objects) during the fall to trap some heat during the cooler days and during the evening.

Place your waterfall and pump on a lower setting, if possible, as this will cause the water to be less turbulent and will help raise the temperature. However, if it’s very cold, don’t switch them completely off as the water in the pipes can freeze quite quickly.

As always, keeping an eye on the water temperature is as important during cooler weather as it is when it’s hot.

If the weather is turning towards fall and winter, it’s important to take precautions before the seasons change completely. This will ensure that you are always in control of the water temperature and keeping your koi comfortable.

How To Prepare Your Koi Pond For Winter

When the seasons change — especially if you live in an area where these changes are very pronounced — it’s important to keep a close eye on your koi pond and be ready to put in the effort and work it will take to keep your koi happy and healthy during the colder months.

Prepare a winter tank or pool

If you know that you live in a very cold area and that keeping your pond at the right temperature or ice-free is not really possible, start to ready an aquarium or holding tank for your koi inside (for instance in the garage) before the cold comes instead of rushing through it once the pond is already freezing over.

You can then move your koi before they start to “hibernate”. Koi do not really hibernate – what this means for you and 4 facts you must know are explained here!

Also, we show you in this article what to do with baby koi during the winter.

Use Heating Techniques

You can also build a greenhouse-like enclosure over your pond to keep the water from getting too cold over the winter months or using a solar cover (the kind used for pools) to keep the temperature from getting too cold or fall too quickly.

If you choose this route, you can also quite easily use a koi heater to keep the pond from freezing over. (It’s very important to still run your air pump even if the water is freezing over.)

Get Rid Of Parasites

Ensure that your koi and your pond are parasite-free before the cold weather hits. It’s good to treat your pond in fall.

During winter the pathogens will most likely be dormant, but once the warmer weather comes they can flourish and attack koi that are vulnerable after winter.

Potential Bacterial Threats

Feed your koi antibiotic-medicated food once before winter strikes while the water temperature is still above 65 F. This should take care of any potential bacterial issues.

You should also stop feeding your koi if the water drops below 50 °F. While they’ll still eat, they won’t be able to digest the food properly and that could lead to illness and even death in some fish.

Stop The Waterfall

Either stop or bypass your pond’s waterfall (even if it’s in a greenhouse) as this will lessen the amount of water that is exposed to the cold. This will then help to retain some heat.

Insulate The Plumbing

Insulate your koi pond’s plumbing for winter to help keep the water from freezing inside. While you need to bypass or turn off the pond’s waterfall, you shouldn’t turn off the filters, but keep them running throughout the winter.

Control The Ice

If the pond does freeze over, make sure that the ice doesn’t cover the entire surface and that there is space for fresh air to reach your koi. The ice should also not be too thick — koi may be able to survive for times at low temperatures but not literally being frozen!

Stop and empty the water pump and filter but keep the air pump running.

Keep An Eye On Your Fish

Watch your fish closely for any that do not act like the other fish as they may be ill or injured and need to be quarantined.

Do Water Tests

Test the water often and especially if there are fish who seem out of character. You may have a problem with too high levels of nitrite or ammonia and will need to do a major water change of up to 40% to correct it. You should, if this is the case, also stop feeding your koi.

As you can see, all the steps to keep the correct water temperature for your koi fish means keeping the temperature as stable as possible to keep your koi as comfortable as possible.

Comfortable koi means that they won’t be stressed and as prone to disease, giving you many more happy years with your koi!