First, stay on top of your water quality. Good water is the key to healthy koi. Don't let your weekly water changes slide, and remember that you want stable pH. Since most of us rely on the city water supply and not well water, it is important to keep pond water buffered, as city water has very little carbonate. Here in Bothell, tap water contains, usually, 35.8 ppm carbonates, so buffering has to be added. I prefer baking soda, as I know I can quickly and routinely keep my carbonate hardness (or KH), above 80 ppm, and usually at 107.4, which I know will keep my pH at 8.
I would urge everyone as we go into fall and winter, to keep a diary or chart of your test results, temperatures, dates of water changes, fish behavior, etc. Personally, I think it is fun to chart this information, and if you suddenly see changes in pH, you know you have to find out what is happening. For example, if you notice a sudden appearance of ammonia, you have to ask "why?", and then sleuth it out.
Another very important concern: do not add products to your pond to treat fish if you do not know what you are treating. If your fish have costia and you instead treat it for flukes, your fish will probably be dead by the time you figure out your mistake. It is important in these cases to learn how to use a microscope. This will save money and fish over the long haul. There are many products out there, and some businesses love to sell things to anxious pond owners. But please ask yourself, "do I know why I am adding this to the water?". Refrain from a compulsion to "fix" if you don't know the problem. For example, be careful when someone tells you to add salt to your water. If you need to treat for Ich or chilodinella, and your salt level is 5 ppt, you won't be able to add a formalin/malachite green product like Proform C or Ich-X, which would treat the fish until you get the salt level down, or you will likely kill your fish. This will also add further stress on your fish with huge water changes so that you can add the right chemical. A quick "scrape and scope" would be the first thing to do.
Also, now is the time to rinse filters (with pond water), vacuum the pond, check on valves, fittings, and the filter pads on the air pump. As the weather changes, it will also soon be time to trim back water lilies and remove water hyacinths.