Getting Ponds and Koi Ready for Wintertime

Getting Ponds and Koi Ready for Wintertime

By Bonnie Hale Guest Author


If you must move fish inside for the winter, do this gradually. Move a few Koi inside to an aquarium or stock tank that has been set up in advance using pond water. Make sure that their winter home is large enough to accommodate them without over crowding them. Figure 1″ of fish length for every square foot of water surface. If you have a lot of extra filtration or lots of surface coverage with plants, you can bump that to 2 or 3 ” of fish for each square foot of water surface. Bring in as many plants as practical so that you have good water surface coverage. A tank, pond or aquarium that has a nice amount on surface plants will make for happier Koi. Rarely, if ever does a fish “jump” if you have good plant coverage. When they get scared they will hide under the plants. If there are no plants and the fish gets scared, he will jump out. I have never had a fish “jump out” in ponds or tanks that have good water surface coverage of plants!! But over the years I have lost my share of jumpers to tanks that do not have plants. I now keep netting over my non-plant tanks. Place lights above tank to support plants. Use a timer on the lights so that they have 10-12 hours of light a day.
If practical, move your pump and filter too so that Koi will be in almost identical situation as they were in outside. Do not move all fish inside in a new tank all at one time or they may all die. Plan to do this task early enough in the fall so that you can add a few fish every week. Monitor ammonia, pH, nitrite, and nitrate very carefully and take corrective measures if these measurements go out of balance. Feed very sparingly daily so that you do not get an ammonia spike. After a few weeks, increase feedings. My one stock tank where my favorite Koi spend the winter, get fed 3 or 4 times a day and they grow like weeds all winter long in 70 degree water.
In order for the fish to remain outside for the winter, your pond has to be a minimum of 18″ deep. If less than that, see article above about moving Koi inside.
Leave the pump run all winter or use a de-icer. Move the pump close to the surface of the water by placing it on cinder blocks or upside down clay pots. You want to have the water just bubbling just under the surface of the water. This will keep an open spot so threw which gases can escape. Once the weather gets real cold, disconnect the filter, clean it and store it. Give your filter and pump a thorough cleaning before the frosty weather hits. Store your pump in a bucket of water somewhere where it will not freeze during the winter. This will protect your seals in the pump from drying out during winter storage.
Remove frogs and tadpoles and snails. Snails and tadpoles have a hard time in the winter. Bring snails and tadpoles inside for the winter. Release the frogs (they WILL find your pond in the spring) Check ponds frequently throughout the fall and early winter for frogs and remove them. They will probably not survive in your pond for the winter because they need to burrow into the dirt for the winter. Any experienced pond person will tell you that if you leave frogs in small ponds, you will be scooping their bodies out of your pond next spring. The decay from these dead creatures will wreck havoc with your Koi. Let alone the smell!
If you live in a zone that gets cold but little or no ice during the winter, leave your filter run all winter long. It will be easier to get started in the spring. Clean it thoroughly before it gets too cold. Bacteria will not die if chilled. They will remain dormant and ready to start growing again in the spring when temperatures begin to warm up the pond.

Getting Ponds and Koi Ready for Wintertime
Getting Ponds and Koi Ready for Wintertime

Cover pond with netting or skim pond with skimmer net daily. We now have a skimmer that can be “teed” off of your pond pump. It makes removing leaves a breeze. You simply remove the leaf bag and empty out the leaves! Decaying leaves produce poisonous gases. Leaves are double trouble-solid pollution and chemical pollution. Leaves are much easier to remove in fall as they are dropping then from the bottom of the pond next spring.
Water quality at this time of the year is usually very good. The water should be crystal clear. If it is not, do small water changes every couple of days, so that it is crystal clear. Do not raise or lower the temperature more than 2 degrees at a time, as this will stress the Koi. Add water slowly and watch your thermometer. Once algae has died off the water is usually crystal clear. It is very important to keep it that way by keeping leaves; frost damaged foliage from the plants, and any other debris out of the pond. You want the bottom of the pond as clean as possible. A lot of harmful bacteria live in the fouled water. These specific pathogens are Pseudomonas and Aeromonas. They depend on foul water to attack Koi. They will not be a problem in the winter, but come spring, you had better watch out. Entire populations of fish have been wiped out in March, April & May from Aeromonas and Pseudomonas.
Because cold water holds a lot more oxygen than warm water, this is a very satisfactory time of the year for your fish because of their lower metabolism rate.
Some folks prefer to use a de-icer in their pond rather than leave their pump run all winter. The ones I sell are very energy efficient. The thermostat clicks on when the water temperature reaches 32 degrees and clicks off when it reaches 38 degrees. The deicer will keep an open spot in the ice to allow toxic gases to escape and oxygen to enter. If the pond should completely ice over do not try to open a hole by force. Use a hot tea kettle and place directly on the ice to melt a hole in the ice. My pond froze completely for 2 weeks and the Koi were just fine. Short exposures to total ice coverage did not hurt the fish at all!
Add 0.3% salt to your pond. Figure the total amount of salt that you need for your pond and add 1/3 of the TOTAL amount over a 3-day period. This equates to 3 lbs. per hundred gallons of water. Salt kills 7 of the 10 parasites that attack fish. Yes, the parasites can live in cold water! They will begin attacking the Koi in the spring as the pond starts to thaw and warm up. We had great success curing chilodenella (a parasite) a year ago in early spring using 0.3% salt. It also works great on ICH. The salt will be removed in the spring by water changes when the temperature warms up and you are ready to start adding plants.
Fortunately, bringing the Koi through the winter is fairly easy. I have had a lot of customers tell me that their fish survived the winter with flying colors then died in the spring. One word: BACTERIAL INFECTION. The salt will help tremendously. Salt will help build up extra layers of slime coat to protect the fish and it will kill parasites as previously mention.
The salt to use – Water softener salt. This is what we used. Good old Morton in the yellow bag! And it is cheap. You can use any kind of salt as long as it does not contain iodine or YPS (an
Anti caking agent) Non iodized table salt will work but it is more expensive. Don’t waste your money on expensive sea salt or aquarium salt. I give you my word that what we used is Morton water softener salt and it worked well.

*2019 Note on Salt: Iodine in salt doesn’t hurt the fish. Someone (Doc Johnson) just had to do some calculations on the actual ppb of iodine in the water versus toxicity to biological processes and organisms, prove there’s no iodophore reaction and then test it. Which I did and nothing happens.

Koi do not have stomachs. Secretions in the intestine digest food. Their ability to digest is directly related to water temperature. Mix equal amount of wheat germ with regular food in September. Gradually switch over to fish food containing wheat germ. It is more digestible.
The Koi do not eat when the water drops below 46 degrees. If you continue to feed it will just sink to the bottom where it will decay and pollute the water. Essentially your fish will go to sleep at 46 degrees. Their heart and breathing will slow down to where there is little or no physical movement. You must treat them accordingly.
When we get into the “spring-like” days of January and February it may be tempting to feed your Koi especially if you see them gathering on the surface of the water. Avoid this urge. Use your thermometer and check the temperature. Feed strictly by the thermometer and not the calendar and you will not have any problems. The temperature has to be above 46 degrees. If you load up their bellies with food and we get a fatal cold snap, it could wipe out all of your Koi. They cannot digest food in cold water.
Use a reliable water thermometer
Feed as follows by water temperature

 61 degrees+ Twice daily or more : Mix wheat germ fish food with your regular food
 56 – 60 degrees Once a day discontinue regular food and use just wheat germ food.
 51-56 degrees Two to 3 times a week
Switch to medicated food. Store in the freezer as you will use it again in the spring when pond water temperatures start to go up again The antibiotic will have no effect on your biological filter. Starting and ending the season with medicated food ensures that your fish sleep or wake up without any bacterial infections.
 46-50 degrees Once a week | With medicated food Do not feed again until spring when water temperature reaches 46 degrees…
 < 46 degrees  Do not feed

Watch water temperature in the spring. Reverse above temperature chart and start feeding fish food with medicated food when water temperature hits approximately 46 to 48 degrees. Followed by wheat germ, then wheat germ mixed with regular food.

Monitor ammonia, nitrite, nitrate and pH levels if you turned your pump and filter off for the winter. Be prepared to do water changes if ammonia is present in even the slightest amount.


  • REGULAR FISH FOOD (preferably Optimun/Impact)
    Cover plants if light frost is predicted. You may want to take the tender plants inside on frosty nights so that they can adjust to the inside environment. If weather permits you can move them back out the next day. We usually get a frost that will damage plants. Then we get a few weeks of Indian summer where the plants will continue to grow and help remove ammonia from the water. Covering them on frosty nights will prolong their usefulness and beautify the pond. Indeed, some pond plants have striking fall colors. When we get 40-degree nights, it is time to starting making permanent arrangements for your plants.

Clean up frost damaged foliage by trimming all foliage with shears or pruners even with the top of the pot. Drop the pots all hardy plants to the deepest part of the pond, where they will spend the winter. Move non-hardy plants inside for the winter

If you have a lot of plants that you plan to bring in for the winter, purchase one of those little plastic kiddy pools. Depending on your light situation, you may be able to place it near a real sunny window or sunny room inside for the winter. Or, set it up in a basement or other out of the way area. Add 3 to 4 ” of water to the pool. The plants do not need to have water over the top of the pot for this type of winter storage. If you do not have a sunny location, place plant grow lights over them with a timer and let the plants grow all winter long. Leave the timer on for 10-12 hours a day. You will have loads of plants come next spring!

Health Impact on Koi of VERY Cold Water: (Video)

Dr Erik Johnson is a Marietta, Georgia Veterinarian with a practice in small animal medicine. He graduated from University of Georgia with his Doctorate in 1991. Dr Johnson is the author of several texts on Koi and Pond Fish Health and Disease as well as numerous articles on dog and cat health topics.