Can Koi Fish Change Gender?

koi fish

Koi fish is one of the most popular ornamental fish that most aquarium hobbyists keep as a pet. All koi fish are colourful irrespective of gender. Differentiating them using their colour is almost impossible. Size is also rarely a determinant of sex.

Differentiating koi fish even by an expert using physical characters can be difficult. Up until koi fish reach 4-5years or 12 inches (30cm) long, the difference is not clear.  Before maturity and sexual maturity, koi fish look alike both in size and colour.

Is it possible for your koi fish to change its gender? With both the male and female koi fish having vibrant colours and similar sizes, how then can you differentiate a male and a female koi fish? Relax and read as you learn more about how to sex koi fish and how to differentiate them.

Can Koi Fish Change Gender?

No koi fish can not change gender. Most koi fish look alike both in size and colour up till they reach maturity when their sex organs begin to develop and become prominent then you can easily tell a male from a female.

At 4-5 years of age, koi develops their sexual characteristic. At this point, you can tell a male from a female. Using bother their body, fin, vent, and behaviour it becomes obvious which male and female.

The females are more plumpy due to the eggs in their stomachs and this makes them round while the male’s body is streamlined. When Koi fish become mature, the male can be differentiated with their behavioural pattern of chasing the females for mating.

How To Different A Male And A Female Koi Fish

As much as it’s difficult to tell a male from a female some distinctive characters are specific to one gender. You can differentiate between a male and female koi fish with the following features:

– Skin and Texture

The females are larger than the males, however thing feature can only be used if all the koi are of the same age. The males grow tubercules (white growth) at about 2 years of age.

The tubercules feel gritty like sandpaper around the dorsal region and on the gills plates. They appear during mating and disappear after spawning. The females are always delicate and feel smooth when you touch them.

– Body and Shape

The female appears rounder in the middle due to the eggs in them. This makes the midsection larger it is mostly known as a blim shape. The males have a streamlined torpedo shape due to their inability to produce eggs, unlike the females.

– Fins

The difference is not very pronounced. The male’s pectoral fins are pointy or diamond in shape while that of the female is usually rounded. The fin is more transparent while that of the male is more opaque.

– Vent

The vent is one of the physical distinctive characteristics between the male and female koi located on the underside of the koi. The male’s vent is sunken and concave shaped while that of the female is protruding and convex shaped. The female vent has a pinkish appearance, unlike the male. 

– Color Development

There is a difference in the pace at which a color develops between male and female koi fish of similar breed. The male color develops faster than that of the female. This will make the female’s color look lighter or pale when compared.

Some breeders can use this to differentiate but as a new hobbyist, it is not a reliable way because the color of koi can be influenced by many other fa toes including growth, environment, diets, and some diseases. 

The body and fin can help you identify a koi gender but one of the best ways is to check the vent and feel to check for the presence of an egg.

– Behavioral Difference

The period of mating is one good way of differentiating their gender. Similar to almost all animals, the male koi chases the female. This way, you can easily tell the male from the female.

When Can I Sex My  Koi Fish?

Most time when your koi is about 30cm or above in length then it is sexually mature. But from a health point, koi fish should not be allowed to mate until they are 4-5 years of age. At this point, it is safe for them to mate and reproduce.

Maturity differs according to the variety/ specie of koi fish you are buying but it takes approximately 2-3 years for koi fish to grow to 30cm (12 inches). This is the length for its maturity in the presence of a conducive environment and nutritious food.

People who are experienced in breeding koi can sex them even before they are sexually mature. But for a new hobbyist or breeder, it is advisable to wait for them to mature to be able to sex them correctly.

At that point, their morphological features are prominent so it won’t be much of a hard task to sex and there is little to no chance of mistake. When they are too small, there is always going to be a mistake

What Is The Ratio Of Male to Female Fish That Can Stay In A Pond?

Having too many female koi in your pond will lead to an overcrowded tank. The ratio of male to female koi in a pond will differ depending on your reason for investing in them. Male koi fish can be quite aggressive when chasing a female koi fish during the spawning period and this can stress the female.

Having more males in a group than females can be stressful to your female koi fish especially when she is smaller than the males. Larger females can be paired with smaller males to reduce the chances of getting stressed or injured.

You can pair them in the ratio of 2 males to a female so that the female doesn’t get overwhelmed. The majority of the fingerlings get consumed by their parent or other fish. To prevent this you may need to provide a new home for the many baby koi.

If you do not have any intention of breeding koi at all, you can opt for an all-male or all-female pond. Getting all-male will be cheaper to invest than all-females because a single mature female koi cost almost twice or more the price of a single male koi fish.


Differentiating koi fish can be difficult even for breeders up until they become sexually mature. Before their maturity, they are all similar in size, shape, and color, this makes it hard to tell their gender. T

hey become sexually mature at 4-5 years of age, and at this age, they are mostly about 30cm (12 inches) I’m length. At this stage, you can differentiate their gender by looking at their sexual characteristics which are the shape of their vent, their body, texture, fin, and behaviors.

With the characteristics mentioned in this article, you can easily sex your koi fish for breeding, and if you do not want them to breed, you can separate the male koi fish from the female koi fish.

Written by Justin Michaels

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