For what it's worth, at least one member has a nice pond with goldfish. Mike W. does not feed them at all! They seem to do fine.
No one feeds fish in the wild, but sure if ones pond is tiny it would make sense.
I buy my pond food from Arthur Aquatics. I get a 50 or 60 lb bag!
I have Goldie's, Shubunkins and Comets. Red Ear Sliders too. All are doing nicely!
I do feed some lettuce and other veggies at times and they do eat it with gusto.
I have a bumper crop of cukes so I tried that , it disappeared!