How to Know If Koi Fish Is Pregnant?

Koi fish, both males and females, will display a high amount of activity during the spawning period. However, it is important to understand the reasons behind their actions.

Read on to find out how to know if Koi fish is pregnant and what to do at various stages.

Pre-Spawning Behavior of Koi Fish

It is extremely important to know how your Koi fish is behaving before their breeding starts. This will allow you to be aware of any dropped eggs.

You can even put those females who are carrying eggs separate if you want to avoid the eggs being fertilized.

Unfortunately, the pregnant female Koi fish is a target of aggressive behavior from male Koi fish. Males will keep bumping against them to make them drop the eggs they are carrying.

Hence, if you see any female Koi fish which are a little weak or injured, you may want to separate them.

Koi fish are also cannibalistic. They may even go as far as eating their own eggs or young. You will have to be on guard if you want to protect them.

Before spawning, female Koi fish will have a voracious appetite and will even look for food elsewhere after a feeding session. They will also hide themselves in plants and rocks.

Males, on the other hand, will chase the females around the Koi pond or tank. All Koi fish will be found in a much more active state.

Signs of Koi Fish Pregnancy

It may get tricky to differentiate between a plump female Koi fish and a pregnant one. The main difference is in the size of the bump.

Age and Size

The older a Koi fish is, the bigger in size they are. And naturally, the more sexually mature they would be.

Female Koi fish attain sexual maturity when they are between three to five years old. At these ages, they probably grow to around 10 inches in length.

Considering this, if you see a female Koi fish who is rather plump but isn’t 10 inches long, it may mean she was just eating a lot.

On the other hand, a plump female Koi fish which is 10 inches long at the minimum, may indeed be pregnant.


Appearance is a surefire indicator of Koi fish pregnancy. Female Koi fish carrying eggs will have a bulge, but the bulge will be more towards one particular side.

Pregnant female Koi fish will also be much larger than male Koi fish. Not only are male Koi fish inherently smaller than females, but the pregnancy bulge will give them an additional boost in size.

Another point to consider is that a female Koi fish shorter than 10 inches but appearing pregnant may, in fact, be suffering from a disease. Or that she may be a victim of parasites.

The behavior of Male Koi Fish

As mentioned before, pregnant female Koi fish tend to hide in plants and rocks. If you are unable to spot the females, look towards the males.

In the pre-spawning period, the male Koi fish will become highly active and will often form pairs with each other. This is a good sign to know that the females in the tank are full of eggs.

Male Koi fish pairings are formed so that both of them can bump against female Koi fish on both of her sides. This will make her drop her eggs faster.

The behavior of Female Koi Fish

Female Koi fish themselves show rather unusual behavior in the pre-spawning period.

When the female Koi fish is pregnant, she will actively look for places where she can lay her eggs. These places can be rocks, plants, or pond sides. Once she has found a suitable place, she will start cleaning it.

Cleaning the area makes it much easier for the eggs to stick. Unfortunately, the eggs don’t always survive. Koi fish may often mistake their own eggs for food and eat them.

How to Breed Koi Fish

In order to breed Koi fish, you will need to follow the female Koi fish to know where they have dropped their eggs.

When those eggs are fertilized by males, you can take those eggs and place them in a separate area to protect them from their parents.

Once you have separated the eggs, place them in a different pond used exclusively for breeding. This will also give the female Koi fish some peace from her male counterparts.

How to Prevent Koi Fish Breeding

There can be any reason why you want to prevent your Koi fish from breeding. Maybe you are just not interested in it, or perhaps your Koi pond already has too many occupants.

Regardless of your reason, you can prevent Koi fish from breeding by separating the females from the males early on.

You can even control the water temperature of your Koi pond or fish tank. Koi fish breed in warmer temperatures; hence, you should lower it.

How Long Is a Koi Fish Pregnant?

The entire spawning period of a Koi fish can last up to a week or even more.

The female Koi fish will look for certain environmental factors that will affect her hormonal conditions so she can release her eggs.

However, you need to be on alert during the spawning period. During this, there is a lot of chaotic activity in the fish tank or pond.

Best Setup for Koi Fish Breeding


If you want to observe the whole pregnancy process of Koi fish as nature has intended, it is best to have a pond setup. However, this may reduce the number of Koi fry that will survive.

Koi fish can lay several thousands of eggs. Unfortunately, many of them are eaten up by other fish in the pond and even by Koi fish themselves. This culling process is quite natural.

To give them a higher chance of survival, you can add various plants or decorations to the pond so the fry can live.

Fish Tank

Another option is to move the Koi eggs from the pond to a tank. If you want to have an indoor fish tank setup, have one for breeding and a hospital tank for recovery of females after their breeding process.

However, fish tanks require extra equipment like filtration, water pumps, air stones, and more.

If you want to induce the spawning period, you will have to simulate the springtime conditions. Try to increase the duration of the light cycle and the water temperature slightly.

As an added feature, you can hang thin strips of nylon ropes. These are particularly useful as they give the female Koi fish a place to lay their eggs. You can also remove those eggs very easily.

In a fish tank, you should also monitor the oxygen levels and ensure they are kept optimum for Koi fish mating. If you face any problem with this, an air stone can be extremely valuable.

Water Requirements

The water parameters for spawning Koi fish are the same for both indoors and outdoors. The two main aspects are the ammonia levels and the temperature of the water.

Ideally speaking, your water should be at a temperature between 65 degrees Fahrenheit to 75 degrees Fahrenheit. There should be no ammonia in the water, as even the slightest amount can be fatal.

To make sure the water requirements are met continuously, it is a good idea to have a quality testing kit. You should also change 20% to 30% of the water of your tank regularly if you want to keep it clean.

How do Koi Fish Eggs Look?

Koi fish eggs are quite small in size, measuring a few millimeters in diameter. They are mostly round and translucent and can stick to most surfaces.

When these eggs are fertilized, and begin to develop, you should be able to see a brownish, seed-like shape in the eggs with black dots. These will be the eyes of the Koi fry.

Eggs that are not fertilized will only serve to lower the water quality through decay. These eggs will have a milky-white appearance.

You should immediately remove those unsuitable eggs from the pond or fish tank.

How to Care for Koi Fish Fry

Fertilized eggs should take a maximum of a week to fully hatch.

In the beginning, the fry will prefer to stay in the egg sacs to get the required nutrients. After two or three days, the fry will start swimming around the pond or tank.

Koi fry are not able to swim well in the first few days. They will want to hide among the décor and various aquatic plants.

This is the reason why it is recommended to separate the Koi eggs into a different pond or tank. This guarantees complete protection and safety for them.

Once your Koi fry has gotten used to swimming around, you should feed them four to five times a day. It is best to give them liquid food early on and crushed flake food later.

You can also feed them baby brine shrimp or larvae once the fry has started to grow.

Koi fry should be kept in the breeding tank until they have grown to three inches in length. This happens when they are four to six months old.


Breeding Koi fish requires a lot of care from the Koi fish owner. However, it is indeed a rewarding experience.

The process itself isn’t difficult; it just needs constant attention from you.

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