Koi and Goldfish may look very similar at first sight. They are actually two different species and require different care and attention. This is why it is important to understand the differences and what you need be prepared if you wish to have koi or goldfish together.
We have listed the ten most important difference that will help you to be confident and prepared to handle Koi or a Goldfish.
Let dive in without any further ado.
Today’s koi and goldfish varieties arose from breeding efforts that began in China during the Tang Dynasty (618–907 BCE).
The Prussian carp (Carassius auratus gibelio) was a food fish kept in ponds. The fish were not exciting to look at, having a drab olive green color. But pond-keepers noticed some fish had unusual orange and gold coloration.
These mutant fish were thrown into smaller ponds and kept as pets and curiosities.
These fish continued to mate and produce more colorful offspring.
These mutations ultimately produced the smaller, colorful Carassius auratus, which we call goldfish. Koi lineage has proven difficult to sort out, primarily due to today’s modern genetics science.
Until recently, koi were believed to have been bred from the common carp (Cyprinus carpio). Early records from the Jin Dynasty indicate amur carp (Cyprinus rubrofuscus), an aquacultured food fish, were developing colors. Japanese selective breeding of colored Amur carp began in the 1820s. These fish, called nishikigoi, are what we call koi today.
Young koi offered for sale as small as three inches.
They’ll grow to an average length of three feet.
Koi have the typical elongated body like their carp ancestors. Their mouth faces downward toward, making it easy to forage along the bottom of a muddy pond.
Koi have two pairs of barbels on each side of their mouth. Barbel is Latin for “little beard.”
Barbels are sensory organs, used for detecting food by taste. Koi use their barbels to sense food buried in muddy sediments.
Goldfish varieties come in a variety of shapes. Young comet, shubunkin and common goldfish look very similar to koi.
But they won’t grow nearly as long as koi. These goldfish can only reach a length of 12 inches.
Fancy goldfish, Black Moors, Fantails, Orandas, and Ryukins, have an egg-shaped body. Fully mature specimens can grow up to 10 inches in length.
Their flowing fins look beautiful but aren’t good for swimming.
Koi breeders have worked for centuries to develop specific colors and color patterns on the fish.
The main colors of koi are white, black, blue, red, yellow, and cream.
Breeders have created specific color combinations and patterns and given them names.
Taisho Sanshokua is a white skinned koi with a red and black pattern.
A white skinned koi with a red pattern is called Kohaku.
Goshiki is a mostly black koi with red, white, brown, and blue accents. There are over 23 koi categories.
Goldfish varieties do not have the complex colors and patterns like koi.
The main colors are black, red and yellow pigments. White has no pigment in the skin. There are also gray, brown and blue categories that are caused by variations in the pigment ratios.
Another relevant difference between koi fish and goldfish are the fins.
Koi fin shapes look like carp fins, as basic as a fish can be.
However, there is a variety called butterfly koi. Butterfly koi have longer, flowing fins with a variety of color patterns.
The wild long finned koi were discovered in Indonesian drainage ditches. A US based fish farm began experimental breeding with these mutations and eventually developed the colorful, long-finned butterfly types we have today.
Koi purists don’t consider butterfly koi to be legitimate koi fish. Many backyard enthusiasts like the look of this type of koi.
Goldfish have several tail types.
Single tailed or fan tailed goldfish have a single caudal fin and anal fin. Common goldfish have single tails, which give them strong swimming ability.
Shubunkins and Comets have a deeper lobed, somewhat longer tail than common goldfish.
Double-tailed varieties, also known as fancy goldfish, have two caudal and anal fins. Orandas, pearlscale, and other fancies have double tails.
Japanese Ryukins have three or four-lobed tail fins.
The oldest known koi lived 226 years in Japan. The fish was named Hanako which means “flower girl”. The fish died on July 7, 1977 at 226 years of age.
She was born in 1751, making Hanako older than the United States.
The average lifespan is 50 years, with some koi living longer. A lot depends on the care of the fish and perhaps genetics.
Goldfish have a shorter lifespan than koi. Tish, a common goldfish, lived 43 years in large bowl. Fancy goldfish live about fifteen years. The lifespan of goldfish is going to be controlled mainly by water quality, proper feeding and proper aquarium care.
The koi’s mouth is relatively large and extends down, enabling the fish to dig in the mud and sediment.
Koi have molar like pharyngeal teeth used to grind natural food and commercial feeds.
Koi do not have a traditional stomach. Food passes from the mouth and into the intestine.
By nature, koi are omnivorous, eating a variety of foods including planktonic crustaceans, insects, worms, the tender water plants and fish eggs.
Koi will also eat small fish if they get too close to their mouth! In our ponds, koi are fed prepared diets in the form of pellets and food sticks.
Koi food is made in a variety of sizes tailored for different size fish. Koi crunch the food into smaller particles with their teeth, located deeper in the throat area.
Goldfish are also omnivores. They’ll eat small worms, crustaceans, bits of plant matter and fish eggs.
Their small mouth limits what live foods they can ingest.
Goldfish have pharyngeal teeth to grind up their food. Like koi, goldfish don’t have a stomach. Food is rapidly passed on to the intestines for digestion and absorption of nutrients.
Goldfish will eat flake and small pelleted foods. They will also eat frozen and live brine shrimp and worms.
We have a complete goldfish feeding guide if you want to know more.
7. Life in an Aquarium
Another very important difference between koi and gold fish is the space they require.
Koi grow to a very large size.
Unless you have a very large aquarium, over 100 gallons, you can’t keep a koi in your tank.
They need room to swim and explore their surroundings. Stuffing a fish capable of reaching three feet in length in a small aquarium is not wise or proper care for the fish.
An outdoor (or indoor) pond is the best environment for one or more koi.
The pond gives the koi plenty of room to swim or hover below the surface.
Koi are best viewed from above, so the pond is the perfect location to see the fish’s beauty.
Goldfish thrive in an aquarium. But do not make the mistake to put them in a bowl or in small tank. They require space to grow.The fancy types are best kept apart from tropical fish.
Their slow swimmers and won’t be able to compete for food.
Fancy, flowing goldfish fins attract attention and invite them to nip the fins and body. Shubunkins, Comets and common goldfish will be happy in an aquarium or pond. They’re tougher than the fancies and will spawn every season outdoors.
8. Compatibility with plants
Koi love to eat plants.
They have rather large, powerful mouths that can ingest small aquatic pond plants. Koi have a strong mouth that will snap and crunch lily leaves and the rots of floating plants. Their inquisitive nature leads them to dig around potted plants, spilling the soil and uprooting the plants.
In the pond, goldfish will leave plants alone. Most goldfish enthusiasts use plastic plants in their display aquariums.
It is easier in most cases to use plastic plants because goldfish to like to dig in the gravel and uproot live plants.
Sick koi (and goldfish) pose a challenge for pond owners. If the fish can’t be moved into a smaller hospital pond, the entire pond must be treated with the medication.
This can be very expensive and stress the pond’s ecological balance. The ideal situation is to move sick fish into a smaller treatment tank or pool.
Once the fish has recovered, it can be moved back into the pond.
Goldfish aquariums are much easier to treat because the water volume is much less than most outdoor ponds. It will require less medication and water changes when the treatment is complete.
10. Treating diseases
Koi are powerful fish, even when they’re small. You’ll need a sturdy net to capture them. If your working in a larger pond, the net will need to have a long handle and wide net to reach the fish.
If you need to grasp the koi, be prepared to hold onto the squirming fish with both hands!
Goldfish are not as large and can be removed from the aquarium with a regular aquarium net. You’ll be able to cradle a fancy goldfish with one or two hands.
Shubunkins, Comets and common goldfish have longer bodies and will flop around more than fancy goldfish.