by Ray Jordan
Texas Koi and Fancy Goldfish Society

Foreword: This is the final part of a four-part series of articles about care of koi in warmer climates. The information in this article is a combination of my experience as a koi hobbyist, articles I have read, and lectures attended, and my experience as a koi health advisor. Koi are best raised as a four season animal. Perpetual summer or winter temperatures have their own unique disadvantages on the growth and/or health of koi. In some colder climates where koi are kept, pond heaters are necessary to provide a more healthy environment and acceptable growth. Likewise in hotter climates without cooler seasons, efforts to moderate the effects of warm water including methods to lower water temperature at least part of the year are beneficial. Experiencing at least some seasonal variations in terms of temperature and feeding techniques can improve the health and beauty of your koi. Advice on the best care of your koi depends on many factors including your local climate, source water perimeters, pond and filter system, and personal preferences.

It is hard to believe that winter will be shortly upon us, maybe. Unlike colder climates where it is necessary to heat ponds or warmer climates where summerlike temperatures occur year around, our local climate has it’s own unique challenges this time of year.

Many fish problems that seem to be spring related actually have their orgins because of winter stresses and problems created months earlier. It is important to remember that the temperature at which carp/koi are most efficient is ~ 68-78F. One of the most important concepts in maintaining healthy koi requires understanding the effects that temperature has on oxygen content of water.

Temperature/Fresh Water
°F ppm/sea level @ 80% saturation
50 9.0
55 8.5
60 8.0
65 7.5
70 7.1
75 6.7
80 6.4
85 6.1
90 5.7



First, temperature affects the ability of water to hold dissolved oxygen.

Second, the rate of metabolism doubles for every 10 degree centigrade temperature change (18 degrees F) in cold blooded animals. This means the metabolism decreases by 50% for each drop in temperature of 18F and conversely doubles for every increase of 18F. For winter we need to remember that decreasing water temperature from 70F to 52F means that the requirements of fish, bio-filtration, bacteria, algae, in fact all living things are reduced by 50% for nutrients, oxygen, and everything else. At the same time the oxygen content of the system would go up not only because of cooler water’s ability to dissolve more oxygen but also because of the reduction in the oxygen demand on the entire system. If you were to compare the amount of oxygen and the oxygen demand at 52F degrees vs. 88F degrees the difference is even more significant. At 88F your pond oxygen level would be about 1/3 less however the oxygen demand would be 4 fold greater than at 52F. Is it any wonder that a pond’s water quality and koi look so much better during the relatively mild winters that usually bless our area. Another factor to consider is that the immune system in koi become less efficient as water temperature falls below 65F and is very weak below 55F degrees. Remember that the beneficial bacteria in your biological filter become less efficient at converting ammonia, nitrite, and other wastes as your pond water drops below 65F as well.There are some excellent cooler water advantages. Cooler water holds more oxygen which is one reason our koi’s colors appear deeper and brighter and their general overall condition seems to improve as long as it doesn’t get too cold or temperatures drop too quickly which can be stressful. In fact this is a time of the year when you usually will have the cleanest water and your fish will look to be in peak condition. This should be a reminder of just how great your fish can look given optimal conditions. In order for your fish and water to look this nice in the summer you will need to consider implementing some of the suggestions in the summer pond tips article.Most fish parasites and disease that can lead to bacteria, etc. are less active in cooler water. However they are ever present and will cause problems if the fish become stressed or water quality problems occur. One of the significant stressors on our koi are the extreme swings in temperature that can occur in our area. In past years we have had 100F degree and 20F degree days in February! In our area, rapid changing water temperatures can be a bigger problem than the actual low temperatures we usually experience. Who knows what will happen this winter?

So you need to be prepared. Here are some tips to help get your fish through the winter less stressed and off to a stronger healthier start next spring:  

1)      Do a thorough pond and filter cleaning if needed now. If you do not have an efficient bottom drain and a pond free of debris you need to remove any leaves and other pond sludge that might have accumulated before winter arrives. Never stir up the muck in your pond with fish still in the pond. Use a holding tank for the fish while cleaning your pond. It is especially important to remove any acorns that might have fallen into your pond this fall as they can be toxic in large quantities. To save yourself a lot of work you should cover your pond with netting to prevent leaves and acorns from getting into your pond. To make your pond healthier for your koi in the future please install a proper bottom drain to remove debris from your pond before it can decompose into toxic pollution. 

2)      Continue to test your water and do regular water changes. Remember your ponds biological filtration will not work as efficiently in colder temperatures. You can definitely have toxic ammonia and nitrite levels occur in the winter. Continue to change 10%-20% of your water each week while you are feeding your fish. You might be able to decrease the frequency and amount of water changes in relation to the amount of feeding/fasting during colder spells. However, koi continue to release ammonia via their gills even when fasting. So it is important to continue to backwash your filters depending on feeding schedule and water quality parameters and do not forget to use declorinator with every water change using city water. If there are a few weeks of very cold weather without any food being fed I still backwash my filters and do a small water change at least every two weeks at a minimum. If the sludge from your settlement tank smells you need to dump it more often.

3)      Check your pond plumbing. Be careful that water circulates through all pipes at all times. It is tempting to turn off some of your water pumps in the winter. However be aware that water pipes can freeze quickly and then break and drain your pond. Being able to bypass your water fall for a night or two when we get a sudden cold snap but be sure your plumbing is very well insulated or maintain at least some flow to insure the do not freeze and burst.

4)      Keep your water pump, air pump, and filters running all winter. If you can reduce the amount of flow and/or redirect your water return to bypass your waterfall, in the most extreme drops in temperature – please do! Less exposure to rapid cooling air can help to slow rapid pond temperature changes. Deep in-ground ponds with protected filters will change temperature slower than a shallow above ground pond with exposed filters.

5)      Know your pond water temperature. You will need to adjust the number of feedings and amount of food given as the water temperature changes. I suggest reducing the amount of food and number of feedings to twice a day when water temperature stays below 70F and only once every other day when it stays below 60F degrees. Below 60F fed every 2nd or 3rd day. Do not feed fish at all when water temperature stays below 50 to 55F degrees. (I promise they will not starve) This is the hardest part for Martha. Our fish always swim up to us even on very cold days and expect to be fed. Not because they need to eat but because they are conditioned to feed. They will of course eat if offered food, but, if their body temperature is below that necessary to chemically digest food (`50F) food will pass undigested through their body and becomes a source of toxic waste and additional ammonia. And no your bio filter cannot detoxify ammonia very well in very cold weather because the bacteria are also dormant or very inefficient at very low temperatures. It is kinder to stop feeding your fish and have them enjoy healthier better quality water conditions.        If you over feed this winter, when the water warms in the spring the increased organics lurking in your pond are waiting to feed an explosion of bacteria to decompose it. Higher organics can also encourage parasite infestations. More fish are killed by overfeeding than almost anything else except for forgetting to use dechlor when adding water.

6)  Watch the weather forecasts and check your pond water temperature very closely. Remember that South Texas weather can change drastically. In warmer parts of the world koi can be fed year round. In colder areas that have a true four season climate koi can be fasted for the winter and they can be left in a virtual “hibernation” with minimal maintenance. In warmer climates like south and central Texas our winter arrives rapidly but usually only a few days or weeks at a time. This requires us to react to the weather and water temperatures accordingly. One easy and fun pond toy is a digital infrared thermometer. They can be bought at radio shack and will give an instant accurate water temperature reading. Sure beats sticking your hand in frigid water to fish out a regular pond thermometer. 

7) Feed your fish according to water temperature: While our winters vary, on average, our winters are short (late December to late February) with periods of warming between cold fronts that can be fairly extreme. Depending on your pond design and total water volume your pond temperature can drop up to 20F degrees overnight when a major cold front blows in. This is very very stressful to your fish and the beneficial bacteria in your filters. Extreme temperature drops in short periods of time can even kill less hardy koi.

If you can bypass your waterfall on the coldest nights it will help to slow down the rate of temperature drop. Let’s say it is January and your pond temperature is about 60F. A major cold front is due tonight with temperatures expected to drop to the high twenties. It will be critical to bypass your waterfall and stop feeding your fish at least 48 hours before the front hits. This will allow your fish to digest previous feedings and also allow the bio-filters in your pond to process the waste before the temperature drops. Checking your pond water temperature after the front you can see the temperature dropping over several days to near 45F. Then it slowly climbs back into the 50’s over the next week or so. Be conservative about beginning to feed. Once the temperature stabilizes for several days at 55F or higher you can begin feeding very lightly once or twice a week.  All the time watching for when the next cold front will show up and the cycle repeats. If in doubt do not feed. If you are going out of town do not have your fish fed. Better to not feed at all than to over feed. In general most years I do not feed my koi for six to eight weeks during the winter. This is not a continuous fast but one that lasts a week or two interrupted by one or two light feedings followed by another week or two of fasting as the cold fronts march through our area. When I say feed lightly I mean only 2-3 pellets per small to medium size koi once or twice a week. It is worth repeating that it is better to underfeed or not feed at all than to over feed. Here is a chart to help illustrate the concept. The vertical axis is amount fed according to total weight of koi in the pond.

8)   Watch your fish carefully for any that do not swim or act like the other fish. On very cold days they will all be very slow or inactive but they should all act similar. Watch carefully any fish that swims funny or sits on the bottom when others are starting to swim around. Watch for cuts or sores that might develop. Injuries will not heal well in cold water and when we get a few days of warmer temperatures the bad bugs can wake up and cause big problems. If you test and find high ammonia or nitrite do a major water change (30%-40%) and stop feeding. If the signs and symptoms point to parasites catch and scrape a few of your koi and look at the scraping with a microscope. If significant numbers of parasites show up use appropriate treatment for the water temperatures you are experiencing.

9)      Move sick, injured, or just weird acting fish to a heated quarantine/hospital tank. Do not move a fish from very cold water into a heated hospital tank without acclimating them first. If you move a fish too quickly into warmer or colder water they will go into shock and die. If your pond temp is 45F and your heater quarantine tank is 70F I would take a tub and fill it 2/3 full with pond water then add water from the heated q-tank. Check the temperature it should not be more than 10F different than what the fish is used to. Add a small aquarium heater and a air stone. Slowly raise the temperature to within 10 F of the Q-tank overnight, then you can put the fish into the q-tank. After a few weeks and the fish has recovered you can allow the water in your hospital tank to equalize with the outside temperature and then put them back into the pond. Or leave them in the q-tank until spring if there is room.

10)  Steps to control your pond water temperature: In our area few ponds use a heater or greenhouse like pond enclosure. However these methods are very helpful to keep temperature stable. Other helpful strategies are to insulate your pond plumbing, filters, and if you have an above ground pond try to put some type of insulation around the outside exposed walls as well. Remember, you are not trying to keep your koi in perpetual summer temperatures. There is a lot of information that suggests there are advantages to allowing koi to experience all four seasons but keeping rapid temperature swings and extremes a little less stressful will benefit your koi.

The less stress your fish experience through the winter the fewer problems you will have next spring and the rest of the year. I hope in some way these articles have helped you and your koi.       Best wishes to everyone for healthy and a happy winter and spring pond season!