Depends what type of filtering you are referring to. If it's to get rid of their waste(solid) and to circulate water the answer is no. If you want to just keep the water clear and fresh,then I would suggest you plant a lot of oxgenators in the water. A well planted pond will act as a biological filter. Certain plants, such as water lettuce or water chestnut will also act as filters, by absorbing the harmful materials. Depending how many Koi you intend to keep and how big your pond is, I would suggest you fit in a good circulating water pump at the very least. Unfortunately this involves electric.