I have a small 1000 gallon, 4000 liter pond that I stocked with goldfish.
I feed them 5 different foods based upon the water temperature so their gut bacteria are working properly. I have to protect them from predators, like Green heron and raccoons. I work hard to keep the water temperature within their ideal range by adding cold water between 4 and 5 AM when there is no sunlight during the summer and noon to 2 PM in the winter so they get warm water.
I have an elaborate timer set-up that controls the water level perfectly. I spray the water in as a mist like at the vegetable counters in grocery stores to further oxygenate the water and spread it out over a wider area so there are no hot or cold spots. I run 2 air pumps to add oxygen. I run the water through a 10um filter to collect particulates and run a skimmer to clear the surface of debris. All these are running 24/7. The water is crystal clear.
They are doing well but how do they survive in nature?
They seem so fragile.

The first year I had the pond I added water during the day and the fish committed suicide by jumping out of the water. The temperature was too warm and the water too dirty. Why doesn’t that happen to fish in a shallow farm pond?