Many people have asked us over years "Should I add goldfish or KOI (or both) to my pond? The answer is "it depends."
Goldfish are better suited to smaller water gardens and ponds, in 50 - 500 gallon range. Goldfish are extremely hardy and easy to care for, which makes them perfect choice for new pond owner or water gardener.
KOI, on other hand, require a little more knowledge and better water quality in most cases, than goldfish and are better suited to more experienced pond keeper. KOI generally thrive best in ponds over 500 gallons (the bigger - better.)
This is becuase KOI can grow quite large and therefore require more water in pond for proper biological breakdown of waste. KOI are also more expensive (and harder to replace) than goldfish, so this should also be taken into account before filling your new pond full of KOI fish.
Goldfish are an excellent choice for average water garden that is usually also full of a variety of potted plants. Lilies, Lotus, Iris, and submerged annuals - these all do well in a water garden pond with goldfish. Goldfish will not disturb plants, and will enjoy playing around under lily pads without disturbing plants.