Agree, with
@GBBUDD goldfish in even smaller ponds can sustain themselves on natural foods. Now qualifying what is a small pond, is another question, after a certain point to me it is no longer a pond but an outdoor aquarium,( ie like a couple hundred gallons or less) in which case even goldfish will need supplemental feeding. Koi on the other hand even in large ponds are going to need some supplemental feeding, especially as the get larger.
It is truly understanding the needs of the animals we keep, not what is easy for us, but what is best for the living being we are caring for. Coming from the world of reef keeping years ago, there are so many nuances and understanding what fish get along with others, what they eat ( some can be sustained by foods produced by refugiums) the space they need to swim, how sensitive they are to water quality, how these fish are collected and shipped are all of the utmost importance and will lead to your ultimate success or failure. However knowing even then when you think you are doing everything right, things still can go wrong that are just beyond your control and crash your system.