My sister had a pond that had coi in it. Things kept going wrong and the fish would end up dead. She got tired of spending the scratch on the coi, so she started stocking her pond with Goldfish bought at the bait shop.
Without being in a small tank the goldfish get as big as Coi, some of them were fancy goldfish with lots of colors and looked about as good as Coi. And wouldn't you know it, the cheap carp were almost un-killable!
I have seen this too. My grandfather had a small pond before he passed and one year a flock of birds ate them all. He went to Walmart and bought some goldfish that lived for many years. Some grew pretty large and the fancies looked great.