Thank you for the further information.
The fish are certainly not giving us much to go on. The lack of extenal clinical signs of the deceased fish does allow us to rule out skin based disease (ie parasite, bacterial, fungal) and as long as it wasn't distended nor showed red streaking of the fins/tail/flanks then internal bacterial disease (either causing dropsy or sepsis) are also less likely.
If we can rule out primary water parameter issues and we don't have any potential vectors introducing disease (ie new plants, fish), then I am suspicious that you are seeing the declining water temperature impacting the fish. As the temperature drops, fish (especially the most sensitive ones or those with less mass:size ratio) will start to show adverse signs. This can include any with sub-clinical disease succumbing to these diseases, as the colder temperature will negatively affect the robustness of their immune system. Otherwise, even with healthy fish, we often see them linger at the surface since this is where the water is the warmest. We can also see depressed movement, bottom sitting in some (usually in later stages), appetite decline and anorexia (which if the temperature remains low they will stop eating and live off their body stores). This is obviously not ideal since natural over-wintering is hard on fish and often will cause some further mortalities.
Now this temperature is on the borderline, which is why I think only a few fish (those sensitive souls) are showing signs just now. Still I do want to cover overwintering for this pond since it'd be best to be intervening at this stage rather then waiting until the temperature plummets further. Therefore, if possible, we don't want to leave the fish to survive to the best of their ability. Instead, we want to step in and make things easier for him. To do so, we do have a few options; though which is most practical for your fish will depend on your situation, resources, and stocking density of fish. The common approaches we can take include relocating the fish to an indoor pond, set-up an indoor tank (either in the house or garage), heating the pond with a supplemental heat source, or using a combination of aeration and a de-icer to block the pond's ability to settle and eventually freeze.
Now the first two options are self- explanatory and as I have noted before the practicality of this will depend on your resources and the size/number of fish you are managing. This is often an option for smaller ponds or in cases where there fish stock are few or very small.
In regards XXXXX XXXXX a pond, this does typically require the investment of a installing a heating system. While this tends to be the most costly choice for keeping the pond warm over winter at least initially, it is a great option to keep the water temperature stable year round. And for parts of the world where these temperatures are common, this is a good investment to keep your winter fish losses as low as possible. The supplemental warmth will keep the fish comfortable, safe from the dangers of the cold and maintain a stable temperature (via heating) that can boost a fish immune system during this difficult time. As well, just to note, if you are modifying the water temperature to keep the pond warm during winter then the fish will be active as normal. This means they will have a normal rate of metabolism and therefore will have a normal requirement for nutrition and oxygen. As well, they will have a normal rate of waste production and therefore require the same level of care as they would in the warmer times of years. So, a heating system is a good option to keep koi activity as normal as possible, their lives as stable as possible, and avoid the serious stress this type of weather puts on their bodies. That all said, you can gently (specifically gently to avoid any quick change temperature shock) try supplementing the pond with warmer (treated) water at this stage to see if these signs respond and their behaviour normalizes.
Otherwise, while the weather isn’t quite cold enough yet for this, I do want to mention the de-icer/aerating option as well. This is where we allow the pond to react to the cold weather but in a controlled matter. By this I mean, allowing the pond to reach cooler temperatures but managing it to keep the pond as stable as possible and avoid a full freeze. To do this, the first step is to install an aerator or power head at the pond's surface. This will increase oxygen levels in the water but just as importantly it will keep the water moving, mixing and therefore harder to freeze. Further to this, we can install a de-icer to maintain the pond’s temperature ~40 °F. There are a range of de-icers on the market and you can see a comparison of these here (LINK). As well, here (LINK) is a wee video to give you an idea of how these are set-up and useful for our ponds.
Overall, with the lack of disease induced clinical signs and in the face of the declining temperatures, I would be quite suspicious that your fish are showing early stage cold stress. Therefore, while an autopsy of the deceased fish would be ideal to ensure there were no internal diseases we cannot see, I would say that it would be prudent to try to warm the pond at this stage and monitor the fish for normalization. If they respond, then you may want to choose one the above options to stabilize their water temperature and give them the best chance of surviving the winter.
I hope this information is helpful.
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All the best & happy holidays,
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