How to Tell if Koi Eggs are Fertilized

how to tell if koi eggs are fertilized

Raising koi of any age is an interesting and enriching activity that makes for years of fun, excitement, and anticipation.

There is something so special about watching your fish grow and thrive under your care and effort. This is especially true if you get the chance to hatch your koi from eggs.

Whether breeding by choice or hoping to hatch an accidental batch, raising koi from birth is a very wonderful experience that is worth the time, effort, and money that goes into curating the necessary care for baby fish.

Let’s take a look at how to tell if koi eggs are fertilized to help you check the viability of your clutch of eggs and better understand the necessary next few steps in their care. 

The Koi Mating Process

Koi fish most frequently mate when the temperature outside is between sixty-five and seventy degrees Fahrenheit. This time period provides an optimal pond quality to support the eggs and young fry that will eventually hatch, ensuring a higher chance of successful reproduction.

Koi fish are known for being great at producing large quantities of healthy eggs at a time so if you are breeding koi, you are likely going to have a pretty good chance at successfully creating a happy little clutch of eggs. 

To mate your koi, select two that are of different bloodlines and place them into a separate breeding pond or tank. This will be where they eventually lay their eggs and the fry are hatched.

Add a spawn mop or other similar item to help keep the eggs anchored and give the fry something to hold onto. Once they are comfortable, the two fish will likely mate.

It is important to keep in mind that koi mating can be rough, as the male will nudge the female towards the spawn mop to encourage her to lay eggs. The two may even fight or nip at each other.

To help keep them safe, monitor them as they are in the mating pond and always ensure the water is deep enough to prevent them from flopping out and becoming injured or even dying. 

Once the female lays her eggs, the male will fertilize them. Typically after a day, you will be able to tell which eggs are viable and which should be discarded. 

how to tell if koi eggs are fertilized

How to Tell if Koi Eggs are Fertilized

To tell if the koi’s eggs are fertilized, you should first examine them.

Fertilized eggs will appear clear and will even have small dots inside of them, resembling seeds, that are what will soon develop into a baby koi fish. These eggs have a chance at producing fry and should be kept and protected.

Remove the koi from the pond to prevent them from eating the eggs or damaging them and keep an eye on these new babies. 

The unfertilized eggs should be removed. They will appear fuzzy and white and should be taken out as soon as you can, as the unhatched and unfertilized eggs will begin to break down and give off ammonia, which can kill the other eggs.

They can also cause overcrowding and even contribute to the spread of fungal infections or other disorders within the eggs if not removed soon enough so be vigilant and remove them as you notice them. 

What Comes Next?

Once the eggs are fertilized, you get a bit of a break.

Check the water quality often and ensure there is a good flow and movement to the tank or pond and that you have oxygenation and filtration to keep things safe and comfortable. For now, the growing koi fish are living off of their own egg and its contents and will for a few days.

Around three to seven days after fertilization, the eggs will begin to hatch. Shortly before this, you can actually see them swimming in their eggs as they prepare to enter the world for the first time, which is absolutely incredible and definitely worth waiting around to experience. 

When they first hatch, your new koi fry will attach to the side of the tank and stay low in the tank until their muscles are strong enough to help them get to the top and search for food.

They are very weak and still developing so this can take a bit of time. They have to essentially learn how to be a fish and work out their new muscles to enable movement and exploration of their new enclosure!

Be extra careful during this time and do not handle the new babies. They are super cute but they are also incredibly delicate and can be damaged really easily, even with the most gentle of touches.

Do not introduce new koi to the pond until they are stronger and of an appropriate size.

What Do I Do With Fertilized Eggs Found in My Pond?

If you find fertilized eggs in your pond, remove them as soon as you can, discarding unfertilized ones. Place them in an enclosure with the same set-up as a regular breeding pond and let them hatch. Within a few days, you should have koi fry.

The process is very similar, just be sure to not leave them in the pond since the adult koi may decide to eat them or otherwise injure the new fry as they are born.

Caution is always key in ensuring your new baby koi can survive and grow to be strong enough to handle an active pond environment one day!

how to tell if koi eggs are fertilized

A Complex Process Made Simple

The entire breeding process of a koi can be a lot to work with.

They require a lot of moving around and care to ensure a healthy batch of new koi from their mating.

Make an effort to provide a clean, well-maintained breeding area and adequate supervision to ensure the adult fish are compatible and you should do just fine!

Just be sure to check all of the boxes and follow all of the steps and you’ll likely have a nice batch of brand new koi fry in no time!

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