Importance of Filtration
Koi are messy fish; when it comes to housing multiple of these waste-factories, a strong filtration is strongly recommended. However, it is important to know that regardless of whether you use a canister, sump, sponge, under gravel, fluidized bed filter, it does not equate to skipping or forgoing water changes. Filters filter waste; filters will never actually remove them from the water. Much like the filters in an air conditioner, it will eventually clog and can become counter intuitive releasing large amounts of nitrates and can damage your filter system.
However, you can extend the deadline needed for a water change by making use of aqua/hydroponic plants. Do note that koi are herbivorous in nature and will consume any plant within reach of their hungry mouths; sump are recommended for growing plants as they are kept far away from the koi. While plants are very efficient at removing harmful waste and provide you with free organic foods, the plants themselves will begin to show health deficiencies if the water runs out of nutrients needed to convert said waste. At the end of the day, a routine water change will solve these issues.
Important to be aware that filters do not work immediately out of the box as is. It can take weeks or months for the filter to develop beneficial bacteria which convert the highly toxic ammonia all the way down to the, still toxic but not as bad nitrate. When cleaning filter media, it is generally recommended to use dechlorinated or pond/tank water as the chlorine in tap water may or may not decimate the bacteria which took months of buildup. However, if you have another pond or tank setup with bacteria rich filter media, you can indeed have a filter up and running immediately by transferring said media to the new filter.