Many pond keepers come to koi keeping by accident.
I, like so many others started out with a goldfish pond in which I successfully kept a range of different varieties of goldfish. I didn’t like fountains but I liked waterfalls, especially as I knew that these would help aerate the water. So each pond would include one of these features and there was obviously a pump to drive it. Apart from this, none of my goldfish ponds ever had any filtration. For many years the fish were healthy and deaths were extremely rare. This is because a goldfish pond, with plants growing in pots on shelves or in gravel at the bottom, is a close approximation to a natural pond or lake. In the wild, fish swim, eat and excrete their waste products, all in the same water. They don’t poison themselves with their own pollution because naturally occurring bacteria break down the pollutants into food for the plants that grow in these waters. This process is known as the nitrogen cycle. In a well planted goldfish pond, even without filtration, there are sufficient environmental opportunities for the bacteria in the nitrogen cycle to grow on the walls and floor of the pond in sufficient numbers to break down the pollutants that the fish produce into nitrates, which are then used as food for these plants.
There is nothing wrong with this type of pond but the pond keeper should be aware that it isn’t suitable for koi. If you are at this stage of pond keeping and wish to graduate to keeping koi then it would be well to be aware of the differences between a goldfish pond and a koi pond and read widely on the subject before attempting to make the change.