We commonly hear this question and it’s a little bit of a headscratcher. Let’s look into the similarities and differences of koi and goldfish and what space, diet, care and 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 (Carassius carassius), there is new finding that suggests this might not be the whole story. Over millennia, 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 Fish – Cypinus rubrofuscus
Koi, more specifically, nishikigoi, is translated from Japanese as “brocaded carp.” These descendants of the Common carp (Cyprinus carpio) 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.
Koi and Goldfish Species Comparisons
So far, we have two species of carp. On the phylogenetic tree, that’s not too far apart. Although they may differ slightly in their external appearance, their care and keeping is almost identical.
The biggest different between koi and goldfish is the space required to keep them. For goldfish, you are looking at at least 20 gallons per fish. Yes, PER FISH. We have seen single goldfish in their own 100 gallon fish tank. However, koi require 250 gallons per fish, with reproductively-active females requiring 500 gallons per fish. Yes, it takes considerable space and oxygen to grow all those eggs.
Once fully mature, koi and goldfish diets will both require a moderate amount of protein (30-33%), low fat (<6%) and high quality ingredients. The size may vary slightly, but overall, the nutritional composition will be the same. If you have growing, developing or are actively making more fish (spawning), you will need higher fat and protein. Here are our diet recommendations for koi and goldfish.
Yes, pondmates, not tankmates. Koi are WAY too big to keep in a tank, unless you have a ridiculously large one (see above space requirements). In the temperate pond system, here are some recommendations for pondmates.
Caring for Goldfish and Koi
The amount of care required with koi fish and goldfish is almost identical. You will need to regularly test their water chemistry, do your routine maintenance and spend time with them to note any disease present. These are not lawn ornaments, but living and breathing animals that deserve your care and dedication as much as any other pet.
Fancy Goldfish Care
Small side note: fancy goldfish varieties (oranda, rachu, ryukin, pearlscale, etc) tend to have considerably more health issues compared to long-bodied goldfish (common, comet, sarasa and shubunkin). They are more prone to buoyancy issues which may interfere with their swimming ability and, consequently, diet. For these types of goldfish, you may be looking at a smaller space that isn’t too deep to prevent exacerbating these issues. Consult with your veterinarian about specific care for your fancy goldfish.
- 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:
- Cyprinid herpesvirus-1 : Carp pox
- Cyprinid herpesvirus-2 : Goldfish hematopoetic necrosis virus
- 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.