Cold Water Effects on Koi

This article is written by Rosimeri Tran, KHA, to discuss the effects of cold pond water temperature on koi and how the koi are affected by vast water temperature changes.

Cold pond water temperatures in our region that range from approximately 40°F to 45°F during the winter months cause the biological filter (bio-filter) to work below full capacity due to diminished nitrifying bacteria count.

Every koi keeper should also know that even if the koi have not been fed during the cold winter months, ammonia is still present in the pond water. Ammonia is mainly created by koi respiration, and due to cold water temperatures, it is hampered thereby making it less toxic to the koi.

Temperature Ranges & Koi Behavior

At 45°F (7.5°C) koi will begin to come out of winter’s hibernation, swimming a bit more and socializing more, versus just sitting at the bottom of the pond, which is normal during cooler pond water temperatures.

At 50°F (10°C) the koi’s metabolism begins to increase as does its appetite. Feeding low-protein koi food is recommended. Some pond keepers even start out feeding plain Cheerios (in the yellow box), feed sparingly, and not every day. Feed in small intervals throughout the coming weeks, so as not to overwhelm the bio-filter with high ammonia production. Being at this low 50°F water temperature, koi will have a reduced metabolism, and their need for food is also reduced.


At 55°F (13°C) the koi’s immune system is now getting stronger. Feeding can increase now as the koi are more active, have higher metabolism and the bio-filter is more populated with the nitrifying colony. Ammonia production will be increased as well, and due to the water temperature rising, this will create a more toxic environment for the koi.  To prevent problems, it is recommended that hobbyists check their water parameters weekly, as well as perform weekly water changes; this will prevent any issues with the koi and the ammonia spike.

Now previously, I mentioned that during cooler water temperatures, i.e. 45°F to 47°F, ammonia levels would be less toxic. You may ask, “Where did the ammonia come from?” since you stopped feeding after 50°F.   Well, the koi create ammonia from respiration, as well as grazing on the floor and walls of your pond where there is a plentiful supply of algae buildup.

IMPORTANT: Even though you stopped feeding during the cooler water temperatures, you should still be checking water parameters and performing water changes.

  • Ammonia is much more toxic in high pH (alkaline) water, so balancing the water pH will make the water changes more effective.
  • Bio-filter “good” bacteria do not start working until the pond water temperatures hit 55°F and above and hold that temperature consistently.

Cold Water Bloating

Symptoms of cold water bloating are similar to Dropsy, i.e. bulging eyes, raised scales, and overall swelling in the body of the koi. Dropsy is usually caused by a bacterial or viral infection or a parasitic attack on an internal organ, which in turn causes the Koi’s body to react by producing more body fluid.

But another cause of bloating can be attributed to the koi’s “osmotic regulation”, which is the process of maintaining an internal balance of the salt and water in the koi’s body. With that being out of balance due to a vast fluctuation in water temperatures over a short time, i.e. 35°F to 50°F would cause the koi to swell and bloat.

To treat koi with cold water bloating, isolate that koi in a quarantine tank so you can slowly raise the water temperature to 52°F with an aquarium heater and add salt to .3%. You should keep the koi in this state for some time to alleviate this condition. But do not return the koi to the pond until the water temperature in the pond and quarantine tank are within 5 degrees of each other.

Cold Water Fungus

A common cold-water fungus seen in koi ponds during winter is “Saprolegnia” aka SAP. It lives in water temperatures ranging from 37°F to 91°F; it is a freshwater fungus, usually found in brackish water and moist soil.

Symptoms to look for on koi are fluffy cotton growing on koi, once koi is out of water; it looks more like a matted mess of slime. The color may be white or grey, with grey color meaning the presence of bacteria, this color may change over a short period to become brown or even green, indicating the presence of algae which has attached to the cotton growth.

SAP is a secondary invader, meaning that there is another issue impacting the koi’s immune system, such as parasites, bad water quality, and/or spawning, just to name a few. So, to treat SAP you will need to treat both the underlying condition as well as SAP.

Cold Shock

Another dilemma that can happen to koi in the winter is cold shock which occurs when the pond water temperature drops dramatically, even a 20° drop can be detrimental to the koi. During the cold shock, the koi will have cardiac and respiratory failure, become lateral, or lay on its side and look lifeless.

The only hope for the koi is to move it into warmer water, such as putting it in a quarantine tank with an aquarium heater, where you can control the gradual rise in water temperatures to a minimum of 50°F and raise the temperature slowly over the next few days.

Koi kept in ponds with water temperatures in the 50-degree range will suppress their immunological efficiency, but not the efficiency of their parasites. Meaning that when water temperatures hit 50°F and below the koi’s immune system is too weak to fight off cold water parasites, so be aware of any strange koi behavior and keep an eye out for any redness, sores, or flashing of koi.

Information found in Dr. Erik Johnson’s book Koi Health & Disease, and from the K.O.I.