Debatably THE single most important water quality element is kH, simply because the levels of Calcium Carbonate (indicated by kH) in our systems affects literally every other aspect of our systems water quality. Calcium Carbonate in our systems comes from two sources; source water and Koi food, but its used up by literally everything, from our Koi to process their food, to the bacteria in our filters that require kH as a fuel, to facilitate vital nitrification.
When water temperatures are high more kH is used, so by the end of the summer its often the case that kH levels are depleted. Sending our Koi into winter with low kH is a problem. That’s why its a good idea to check yours and top it up using a kH buffer. It needs to be above 5˚dh on the test kits = >100 ppm and it need to stay at that level if you want to avoid problems, not just now but next spring too! 
