The more you feed or the more fish in the tank the more often you need to do water changes with goldfish. I use to do mine weekly because I had more big ones. They are slowly dying off here and there, so my bioload is lower now so I go 10 days.OK, I will do that. I used to change the water twice weekly but kind of got lazy on that the past couple of years.
I like goldfish because they are simple and generally can handle large water changes. Tropical fish can be more touchy.