I dug a pretty good size goldfish pond in my yard about 10 years ago as a present for Mother's Day. Put 4 or 5 goldfish in it. Did not feed them, as the pond was self supporting with insects and edible algae. Soon, there were 10 goldfish, then 30. I felt like I HAD to feed them, as there were a few that died, from hunger I thought. Holy Cow, I had 50 in no time and was feeding all the time. Looked great, but it was out of control.
We had a bad winter. The pond froze, like every year, and in Spring, there were only 10 left. I stopped feeding, and it "stabilized" at 4-6, and has been so for years now.
If we don't fix it with a coherent strategy, nature will.
Mac.