Koi vs Goldfish: The Ideal Fish for Your Aquarium


As these two belong to the ornamental fish species, it is evident that the decision can be confusing. Let’s find out the best aquarium fish that meet your requirements.


Koi and Goldfish belong to the carp family. One of the significant differences between Koi vs Goldfish is that Koi consists of a group of carp fish belonging to the Cyprinid family. Carp is a group of oily freshwater fishes found in various parts of the world. They are usually consumed but are considered as invasive species in parts of Africa, Australia, and the USA.

Koi is an informal group Amur carp (Cyprinus rubrofuscus) known for its brilliant color patterns. It is usually kept as decorative/ornamental fish in outdoor or garden ponds. The Koi originates from parts of Japan and is widely distributed in eastern Asia.

Koi are distinguished by their color, scales, and pattern. Popular color patterns of Koi include black, blue, cream, orange, red, and white. There is an infinite possibility of color variation.

Koi grows larger than Goldfish with a maximum length of 3.5 feet. Because of their size, it is not suggestible for indoor aquariums. Yet you can house them if you can provide a large aquarium with enough room for the fish to swim.

Due to increased breeding techniques, multiple categories of Koi are recognized depending on their color patterns. The standard groups are Kohaku, Taisho Sanshoku, and Showa Sanshoku.

Remember that Koi is a subspecies of Amur Carp that are domesticated for their colors. If Koi is left to breed freely, they will return to their original color within fewer generations.

Goldfish is also a carp but is smaller than Koi. It can grow to a maximum length of one foot. In ancient China, a natural mutation resulted in gold color carp. Since then, selective breeding led to a wide range of varieties in Goldfish.

Unlike Koi, Goldfish vary significantly in its size, shape, fin configuration, and color. You can find various combinations of each feature in a single fish. Popular Goldfish colors include orange, yellow, black, white, red, white, and brown.

Goldfish is extensively studied for its senses, such as vision, hearing, and cognitive abilities. They develop acute social learning skills because of their highly developed intuition. You can observe that the Goldfish can swim to the surface and start mouthing for food when their owner approaches the pond or aquarium.

Due to selective breeding for over 1000 years, there are many varieties of Goldfish. Some fish do not have the 'gold’ color present in the original fish.

Few common types of Goldfish are:

  • Common Goldfish
  • Black Telescope
  • Fantail
  • Oranda
  • Veiltail

Since Goldfish are smaller than Koi, you can house them in an indoor aquarium, and they are trendy indoor aquarium pets. The common Goldfish is a hardy one and is less prone to diseases than other varieties.

With proper care, Koi can survive as long as 50 years and Goldfish for ten years. Their lifespan depends on the level of care you provide. Most goldfishes are victims of space abuse. They are kept in tiny pots with limited space to swim. It affects their metabolism and also makes them susceptible to various diseases.

Koi and Goldfish exhibit sexual dimorphism. This topic is discussed in detail in further sections.


As mentioned earlier, both species exhibit sexual dimorphism. It is a bit difficult to distinguish a male koi from a female. Both genders in Koi tend to be in similar size and color patterns. But with some careful examination, you can differentiate a male Koi from the female.

A male Koi will have a slender or elongated body, whereas females are round. The pectoral fin of males is pointed, and in females, it is rounded. A male Koi will chase the female during the breeding season, and it can be another feature to distinguish between both genders.

You will not have these complications with Goldfish. It is easy to distinguish a male gold from the female. The females are larger than males, and their vents (gills) are more extensive than their counterparts. Another significant difference is the pectoral fins in males are larger, thick, and sharper.

Fertilization is external in Koi and Goldfish. The eggs are released by the females that are fertilized by one or more males. Koi eggs hatch within one week, whereas it takes 48 – 72 hours in Goldfish. After hatching, they undergo several changes and attain their final shape in a year.

Ideal Water Conditions

As Koi is larger than the Goldfish, you must provide a larger pond 1 meter deep. If you live in a place where temperatures reach 10º Centigrade or below, increase the pond’s depth to 1.5 meters. A surface water heater is suggested so that there won’t be any temperature fluctuations.

Koi will relent to hibernating the state in the winter season. They eat very less during this season, and their metabolism is reduced drastically. As you will set up the pond outside, the appropriate depth of the pond will help Koi to hide from predators. As a general rule, you must accommodate four koi fishes per 1000 gallons of water.

A Goldfish can be housed in indoor aquariums. In 10 – 20 gallons water, you can accommodate one-inch Goldfish. Similar to Koi, you must install water temperature controllers if you live in a cold climate.

You must test water pH levels regularly. If you are housing a large number of fishes, there can inevitably be an increase in the Ammonia levels (due to their excretion). A sudden rise in ammonia levels can be fatal to both breeds.

Install a water filtering system for efficient cleaning and maintaining proper pH levels. You can use live plants so that they will absorb the nitrogenous waste from the fish. A water pump is mandatory to check the increase in bioload.  An aerator will help in the appropriate oxygenation of the water. Install a UV clarifier to kill harmful microbes.


As Koi is larger than Goldfish, you need more food. The frequency of feeding remains the same for both; two to four times a day. In winters decrease the amount and frequency of feeding as their metabolism is minimized.

A natural tendency of Goldfish is overfeeding. In a natural habitat, as the food is not always available and the presence of natural selection, Goldfish will feed everything possible. A similar feature is observed in captivity as well. So, refrain from overfeeding your Goldfish.

Do not buy cheap, processed foods. Make sure to provide a balanced diet that has high nutrient value. A balanced diet with stable amounts of carbohydrates and proteins must be fed. For optimal health, 30 – 40% of aquatic sourced protein is recommended.

You must clean the water along with the filters at least once per week. Compared to body size, Goldfish produce excess waste than Koi. So, if you have more fish in your pond or aquarium, it is likely to get dirty soon. Change the water along with cleaning as it helps in maintaining the appropriate pH conditions. This activity must be rigorous and thorough in the spring season.

Can we keep them together

Koi is more massive than Gold, and it needs more space. You can house them in an outdoor pond. The question is, can koi and goldfish live together? I would say why they can’t? Unlike the Betta or Guppy, where you need an ideal match to house them together, there’s no confusion here. 

You can house both of them together in a large aquarium or an outdoor pond. Make sure your house in the right gender. If the Koi and Goldfish reproduce, the resulting offspring is of no value. But if you mix a similar gender of Koi and Goldfish, they will go together well. There is no aggression in both species.

Make sure that you don’t put uncommon, delicate finned Goldfish with Koi. You must provide appropriate space for both the fishes to swim freely, Koi’s vast body will crush the poor Goldies.


If you can provide ideal conditions, both carps are suitable for you. You must consider whether these aquatic pets are suitable for your lifestyle or not? Regular cleaning of the pond/aquarium, providing high-quality food, and proper equipment for the pond are necessary.

Choosing between Koi vs Goldfish can be tricky, but isn’t impossible and they both are fit in one aquarium if ideal conditions are provided.

Have you decided to go with both carps or a single one? Let us know what you’ve chosen. We would love to hear from you.

Archiwum: wrzesień 2020

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