Can koi and goldfish be kept together?

We commonly hear this question and it’s a little bit of a headscratcher. Let’s look into the similarities and differences of these two species and what diseases you should consider between the two.

The Goldfish – Carassus auratus

Among the many variations of goldfish is a common ancestor. Thought to be a distant descendant of the Crucian carp, there is new finding that suggests this might not be the whole story. Over millenia, this species has been bred and cross-bred to create a wide variety of specialty breeds. Here are some of the “goldfish” of the world:

The Koi – Cypinus carpio koi

Koi, more specifically, nishikigoi, is translated from Japanese as “brocaded carp.” These descendents of the Common carp can have a variety of color patterns, scale patterns and fin lengths. There are many koi shows every year where the most beautiful fish are matched against each other to win prizes and bragging rights.

Species Comparisons

So far, we have two species of carp. On the phylogenetic tree, that’s not too far apart.

Disease Comparisons

  • Parasites = all the same
  • Bacteria = all the same
  • Fungus = all the same
  • Viruses = not the same

Koi and goldfish do have different viruses they are susceptible to. Let’s look more closely at the cyprinid herpesviruses:

  1. Cyprinid herpesvirus-1 : Carp pox
  2. Cyprinid herpesvirus-2 : Goldfish hematopoetic necrosis virus
  3. Cyprinid herpesvirus-3 : Koi herpes virus (KHV)

Goldfish can only show signs of CyHV-2, BUT they can carry CyHV-3 and not show clinical signs! So, would this be a good reason not to keep koi with goldfish? Not necessarily. If you have a koi/goldfish pond that has a KHV outbreak, your goldfish will survive, but they will be carriers. This is THE SAME for any koi survivors. Your reaction and planning would be exactly the same.

Simply put, you can keep koi with goldfish. The risk is EXACTLY the same for koi-goldfish, all-koi or all-goldfish systems.

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