How to Choose between Goldfish and KOI
Many people have asked us over the 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 the 50 - 500 gallon range. Goldfish are extremely hardy and easy to care for, which makes them the perfect choice for the new pond owner or water gardener.
KOI, on the other hand, require a little more knowledge and better water quality in most cases, than goldfish and are better suited to the more experienced pond keeper. KOI generally thrive best in ponds over 500 gallons (the bigger - the better.)
This is becuase KOI can grow quite large and therefore require more water in the 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. More considerations...
Goldfish are an excellent choice for the 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 the plants, and will enjoy playing around under the lily pads without disturbing the plants.
Japanese KOI on the other hand, and especially the larger ones, will often create a huge mess out of submerged potted plants. They seem to enjoy 'digging' in the soil of the plants and sometimes even knocking them over. This all leads to added mess in the pond, and can create a real problem for the pond owner.
Generally, it's best to not have submerged plants in large pots, when also keeping KOI. The ideal KOI pond is much deeper than the average water garden, so the necessity for plants to help with water quality and shade is reduced.
However, if you still do want to keep potted plants in your KOI pond, we recommend wrapping netting over the tops of the pots, to keep the fish from digging in the pots. Another thing you can do is to top the pots with 1" of pea gravel, and then larger river stones or similar over that. The KOI will not be able to get past the larger rocks.
As far as mixing Goldfish with KOI, this is fine and very common, we've just tried to highlight the most important differences between the two and between the average water garden and KOI pond. Feel free to experiment with both, and then decide which fish is more to your liking.
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Brett Fogle is the owner of MacArthur Water Gardens and several pond-related websites including macarthurwatergardens.com and pond-filters-online.com. He also publishes a free monthly newsletter called PondStuff! with a reader circulation of over 6,000 pond owners. To sign up for the free newsletter and receive a complimentary 'New Pond Owners Guide' for joining, just visit MacArthur Water Gardens>