While Goldfish descend from carps, they are a non-edible ornamental fish. They are a hardy breed that will produce lots of nitrates for your aquaponics system.
Goldfish can grow in a wide range of conditions, but often die when rapid temperature swings take place. They are not territoria, so cannibalism is rare. The ideal pH range is 7.2-7.6, but goldfish can live within a range of 6.5-8.