It is important to understand the effects of water temperature on the ecology of a pond and the physiology of Koi. The purpose of this article is to explain the changes that occur during the cold winter months, and what we can do to minimize the negative effects of these changes.

By now (late November/early December), the temperatures of non-heated ponds in the Chicago area will most likely be in the mid-40s. Let’s first look at what has happened to this point.

When temperatures dropped below 60 degrees F, the Koi’s immune system had already begun to slow down (at 65 deg F, the immune system operates at about 50% efficiency; at 60 degrees it has dropped to about 20% efficiency).
Bacteria, both good and bad, are both still very active. The fish still have healthy appetites, and the filter is operating at near 100% efficiency. Aeromonas bacteria is also quite happy at this temperature, and this is why we are entering what has been called “Aeromonas Alley”.

At 50 degrees, the koi’s immune system is operating at only about 10% efficiency, while aeromonas is still at about 60%.
Some of the warm water parasites have begun to slow down, but the cold-water parasites such as Costia and Chilodinella, are quite content, and can pose a threat to our now low-immune protected koi. Feeding should be almost nil at this point, although the filter is still capable of converting ammonia to nitrites and nitrites to nitrates, so these toxins should not be a problem.

The next magic number is 45 deg F.
This represents the low end of Aeromonas Alley because most bacteria have slowed to less than 20% effectiveness. The koi, because they are cold-water creatures (poikelotherms), have begun to enter a state of torpor, and are most likely no longer interested in food. They will be inactive most of the time, spending their days swimming slowly at the bottom, where the water is warmest (more on this later).

At 40 degrees, which is very nearly the coldest temperature a koi can survive in, the filter has ceased to function.
Bacteria, both pathogenic and beneficial, are at or near zero percent effectiveness, as is the koi’s immune system. In short, everything, both ecologically and physiologically, has pretty much shut down.

Cold Water and Ammonia

It is important to understand the effect that cold water has on ammonia.
As many of us already know, ammonia becomes less toxic at lower pH levels. What many of us don’t realize, however, is that it also becomes less toxic at lower temperatures. Mother Nature is being considerate here, because, as was explained above, our filters are no longer capable of converting ammonia below about 45 degrees, but our koi will continue to release ammonia thru their gills all winter long, even though they are not eating. Ammonia levels could become lethal under these conditions if this were not the case!

What about nitrites?
Nature is again on our side here. Below about 50 deg F, the koi’s metabolism has slowed, and along with it respiration, and thus decreased opportunity to take up nitrites from the water. Once the filter stops functioning, nitrites will no longer be produced, but until that time, nitrites could still build to dangerous levels if we continue to feed. Again, more good news. Nitrite uptake is easily inhibited by adding salt at a level of only .1%, or about 1 pound per 100 gallons of water.

HOWEVER . . . (sheepish grin here) . . salt in very cold water could mean trouble for our koi!
Fresh water reaches its maximum density at 39 deg F, so at this temperature, it sinks to the bottom. This is why our koi will go to the bottom during the winter, and also why we don’t want to mix our pond water during the winter. If we add salt to water, however, the temperature at which it reaches its maximum density is lowered. Salinity levels much over .1% will lower this maximum density to the range of 35 deg F, which could be lethal to a koi. This fact needs to be considered when adding salt to neutralize the effects of nitrite (this is where water changes come in!!)

Cold Water Pathogens

Most parasites are warm-water creatures, and become dormant or die off when the water drops into the low fifties. Notable exceptions to this are Costia (or ichthyobodo) and Chilodinella. These two parasites can be deadly at temperatures as low as 40 degrees.
The good news here is that these parasites are easily eliminated with salt at .3% concentration (about 3 lbs per 100 gallons), or if you are experienced in the use of Potassium Permanganate, a 2 ppm treatment will do the same. A salt or “PP” treatment would be a good idea at about 60 degrees as the water is dropping in the fall, to kill of these parasites. The same should be repeated in the Spring as the water approaches 60 degrees.

Now about this Aeromonas Alley thing.
We know that it ranges from 60 degrees to 45 degrees F. We know that the reason it exists is that our koi have a lowered immune system while the aeromonas (as well as other bacterial pathogens) are still active in this temperature range. The fact is, this is the single most dangerous time for koi-health related problems.

A number of steps can be taken to reduce the risk of bacterial infection.

Since pathogenic bacteria thrive in high organic environments, we should make every attempt to lower these organic levels by keeping filters clean and minimizing the feeding of our koi in these temperature ranges.

A therapeutic potassium permanganate treatment of 2 ppm, as mentioned above for parasite control, will also greatly reduce the aeromonas population as well as reduce the organic load through oxidation.

Finally, there is a product on the market called Lymnozyme, or Koizyme, which has revolutionized aeromonas control. This product is an enzyme that out competes aeromonas for its nutrient supply, and effectively reduces their numbers to manageable levels when used according to directions. I strongly recommend the use of this product in the Fall and again in the Spring.

Testing and Water Quality

Ponders will all too often ignore water testing during the winter months. This is a big mistake! As stated above, koi will continue to release ammonia even after feeding has stopped. Once the filters have stopped functioning, nitrites will not be a problem, but ammonia should be monitored regularly, and controlled thru water changes as needed. In the Spring, it is a good idea to check for nitrites, as the appearance of rising levels indicate that the filter is starting up.

Another very important test is Carbonate Hardness, or KH. Our ponds are still very much alive during the winter months if the water is over 40 degrees F. This means that fish respiration, nitrification, and photosynthesis are all producing CO2, which is neutralized by the carbonates in the water. If the carbonates become exhausted, the CO2, which forms carbonic acid in the water, will cause a pH crash. Many fish deaths have been caused by this phenomena. For short term control, baking soda can be used to raise pH and replenish the carbonate levels. For long term control, oyster shells or crushed coral can be placed somewhere in the pond where water will flow past it, such as over an air stone or near a pump return. A KH reading of 90 or above should be maintained for adequate buffering. As a back-up test, pH should be checked frequently. If it shows a trend downwards, this in an indication of insufficient carbonate.

Finally, and most importantly, WATER CHANGES. This is the best way to control any and all contaminants in our ponds. We will leave a garden hose on drizzle all through the winter, thus avoiding a freeze up in the hose. This also eliminates the need to add a dechlorinator, but if there is any question about chlorine (some parts of the country use very high levels of chlorine or chloramines in the water), sodium thiosulphate should be added at about one tablespoon per 1000 gallons of water added.

With the information provided in this article, you will hopefully have the knowledge to keep your koi healthy and happy throughout the coming winter months. In the next issue, we will discuss bringing our koi and ponds through the Spring warmup season.

(Authors note: much of the factual information in this article is from a Koi Health Advisor Continuing Education course written by Richard Carlson)

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