Goldfish or Koi in a Garden Pond: What's the Difference Before You Decide?
A pond goldfish (Carassius auratus) and a koi (Cyprinus rubrofuscus) look similar in a photo, but their real needs are nothing alike: a koi grows three to four times larger, lives two to three times longer, and needs a far deeper pond right from the start. Getting the choice wrong condemns one of the two fish to a space too small for its entire life. A study by Fife-Cook and Franks, published in 2021, found that koi actively seek tactile contact with a familiar human (research on Google Scholar), a social trait worth weighing when deciding which species to keep at home. Find our other pointers in the bedding and habitat section.
How large do they really get as adults?
A hardy pond goldfish rarely exceeds 25 to 30 cm. A well-fed koi, on the other hand, commonly passes 40 to 50 cm and can reach 70 to 90 cm in a large pond after a decade or so. This difference is not trivial: it drives everything else, from water volume to filtration power.
What pond volume and depth should you plan for?
For goldfish, a pond of 1,000 to 1,500 litres with 60 to 80 cm of depth already works well, see our guide garden pond for goldfish. For adult koi, aim for several thousand litres and at least 1 to 1.2 m of depth, so they can escape herons and survive winter without the water freezing solid.
Can you keep them together in the same pond?
Both species tolerate similar cold water, as covered in our guide cold water and ideal temperature, so cohabitation is possible on paper. In practice, koi grow faster, feed first, and end up dominating the pond: goldfish hang back and feed less well. It is better to size the pond for koi from day one if you plan to keep any, rather than expanding it in a hurry later.
How different is the budget and lifespan?
A pond goldfish costs a few euros and typically lives 10 to 15 years. A decent-quality koi starts around 15 to 30 euros and can outlive 20 years, sometimes far more for show-quality fish. The pond itself also costs more to equip for koi: a stronger pump, oversized biological filtration, and a liner or shell of greater volume.
Frequently asked questions
Can a koi live in a pond built for goldfish?
Only temporarily, while it is small. An adult koi needs far more volume and depth than a typical goldfish pond provides.
Can goldfish and koi live together long-term?
It is possible if the pond is sized for koi from the start. In a pond that is too small, koi dominate access to food and put goldfish at a disadvantage.
Why does a koi cost more than a goldfish?
Its growth, lifespan and the selective breeding behind its pattern justify a higher price, which climbs further for patterns prized by collectors.
This guide is part of Planète Pets’s Fishkeeping universe. Our advice is general in nature: for any health concern, your veterinarian remains the only reference.