So, now the waters getting cold and your koi are starting to become a little lethargic with the cooler water and that's not a problem, or is it? It normally goes something like this: Water cools off, Koi stop eating, they live on their reserves through the winter, water warms up, immune system compromised due to lack of nourishment, pathogenic bacteria comes on as the water starts to warm, the bacteria is happy cuz' it just entered-