How often do koi fish lay eggs? What tank requirements do you need to meet to raise freshly hatched koi fry to adulthood?
The answers to these questions aren’t just for future koi fish breeders. You can use this information to exponentially grow your koi population on a budget!
A female koi fish will lay eggs annually, usually in late spring (May or June). The process of breeding koi is easier than you think, but there are key stages where reproduction of koi fish in captivity will require your intervention.
In a pond, as koi fish reach sexual maturity, they will attempt to breed without being prompted to.
When temperatures signal them that it’s time to mate, and when water conditions allow it, koi will get busy whether you intervene or not.
Knowing how to give fertilized eggs the best chances of turning into superb adult koi fish takes the guesswork out of the equation.
Leaving eggs/fry to “fend for themselves” is a lottery game that few fishkeepers win when breeding koi.
In this article, we will go over all of the details about koi fish reproduction, including how often they lay eggs, how to breed koi, and how to care for them.
How Often Do Koi Lay Eggs?
Koi fish (Cyprinus rubrofuscus) typically mate and lay eggs once a year for about 6 to 7 years after reaching sexual maturity.
Some breeders manage to prompt pairs of koi to spawn twice annually, but the female koi will lay significantly fewer eggs the second time around.
In ponds, koi fish will spawn when the water temperature stays within the range of 65-70°F.
This usually happens in late spring, May, or June. Although this is the “mating season” of koi fish, the egg-laying/spawning will only last for about 2 to 3 days.
Spawning, and the ritual that precedes it, are quite the show in ponds and aquariums alike.
The males will insistently bump into a female that’s looking “ready” to lay eggs, stimulating her to release the eggs, so they can have the first chance to fertilize them.
A female koi will start looking like a lumpy bag of marbles as she swells up with eggs, ready to lay them on pond plants, spawning mats, or spawning ropes.
If you’re looking to breed koi fish in a breeding tank, you’ll need to mimic the ideal water conditions for koi spawning (more on that later!) and make sure the female has plenty of plants to lay her eggs on.
Koi Fish Care
With a lifespan that ranges wildly, depending on the variety of koi you own, but averaging at 25-35 years living in the best conditions in captivity, koi fish are certainly a commitment-loaded purchase for any fishkeeper.
Koi are easier to care for than other freshwater fish in their size range, and they’re considered hardy fish, withstanding wider-ranging water parameters than most aquarium fish.
Here’s a quick koi fish care sheet:
| Care Level | Medium |
| Temperament | Peaceful |
| Lifespan | 25-35 years |
| Size | Up to 36 inches depending on the variety |
| Minimum Tank Size | 250 gallons (for adult koi fish) |
| pH | 7.0-8.6 |
| Temperature | 65-75°F |
| Diet | Omnivorous |
How To Tell The Difference Between Male And Female Koi Fish
Sexing koi fish is relatively easy, and it becomes easier as your koi age.
You’ll have a harder time identifying the sex of a koi fish at the age of 2, for example, than you would when they’re at the peak of their sexual maturity, around 4-5 years old.
| Female Koi Fish | Male Koi Fish | |
| Body shape | Shorter and rounder body | Longer and slender body |
| Pectoral fins | Rounder, almost translucent | Pointy, opaque, intensely colored |
| Vent appearance (anus) | Transverse slit running across the slightly protruding vent | Rounder vent |
| Physical changes during mating |
Will become rounder and chunkier. | Will develop sandpaper-textured white tubercles on the side of their face and on their pectoral fins. |
See how to easily identify the sex of a koi fish here:
Koi Fish Reproduction And Breeding
Figuring the basics of koi fish breeding in captivity and the particularities of their reproduction process is a must if you want to be proactive about increasing your population of koi fish.
These basics are also good to know if you unintentionally end up having to care for koi fry from your sexually mature koi.
Sexual maturity
Koi fish are considered sexually mature when they are 2 years old, but it’s actually not a good idea to have them breed that early.
Most koi varieties should reach the peak of their sexual maturity when they’re around 3-4 years old.
That’s the age when female koi will start laying the greatest number of eggs, and the resulting fry will start having better chances of making it into adulthood.
Something to keep in mind is that although koi fish have outstanding longevity, with some Japanese koi living well into their 50s, they will typically stop mating once they reach the age of 7 years old.
How many eggs do koi fish lay?
A 2-pound sexually mature koi in her prime can lay up to 100,000 eggs during a breeding season.
Professional koi breeders usually estimate the number of eggs by monitoring a female koi’s weight.
For every 1 pound of body weight, a female koi can lay 50,000 eggs.
This means that a 10-pound koi fish would be physically able to lay 1 million eggs.
That number might sound outrageous, but keep in mind that only 60% of those eggs will manage to pass the hatching phase, and even fewer of them will have reasonable chances of survival as fry.
How often do koi fish spawn?
A female koi fish will lay eggs annually, usually in late spring (May or June).
Spawning is usually triggered by external factors such as:
- Water temperature stays within the range of 65-70°F;
- Typical day length for late spring (approximately 12 hours);
- Water conditions are ideal for mating.
A day before the main event (spawning), you’ll notice male koi fish following a female insistently.
Female koi will look rounder and chunkier as she approaches the egg-laying stage of koi reproduction.
Spawning pattern
Breeding koi fish in captivity usually follows a 3-stage pattern:
- Pre-spawning
A day before the female lays the eggs, male koi will engage in a relatively short-lived “courtship” ritual.
If you’re keeping a pair of sexually mature koi in an aquarium, the male will follow the females and bump into her intently.
If you’re breeding koi in a pond, one or more males will chase the rounder female for an entire day, and even in the morning of the main event, pushing her towards the more densely planted areas of the pond.
- Spawning
When ready, the female koi will willingly head to the shallow edges of a pond and release her eggs.
In an aquarium, the female will lay eggs wherever she spots more vegetation.
The eggs will attach themselves to the first surface they encounter.
To make it easier for you to remove the eggs, it’s highly recommended that you provide your female koi with a spawning rope, securing it loosely in the breeding tank.
If you want to keep the process all-natural, you can prep ahead by introducing hornwort into your koi breeding tank. Y
You can plant it into the substrate or leave it as a floating aquatic plant.
- Egg fertilization
With the male following close-by during spawning, fertilization happens very quickly.
As soon as the female lays the eggs, the male koi will start to release sperm onto hundreds of thousands of eggs.
This entire process can make the water inside your tank or pond look extremely cloudy, so there’s no need to panic if you notice a change in water color.
How To Take Care Of Koi Fish Eggs
Getting koi fish to mate boils down to the perfect timing: season-wise and sexual maturity-wise.
Once the female lays her eggs and the male fertilizes them, it’s time for you to step in and take the eggs out of the breeding tank/pond.
Some eggs might be lucky enough to make it through to the hatching stage without your intervention, but if you are intently breeding koi fish, you need to take care of the eggs and later the fry.
Moving the eggs to a nursery tank in order to prevent adult koi from eating them is the first step you need to take.
What do koi fish eggs look like?
Koi fish eggs measure a few millimeters in diameter, starting out as minuscule bead-like dots.
If you’re actively trying to breed koi, you’ll see them covering spawning mops/ropes or sticking to plants in the shallower parts of your pond.
They should be a brownish color, almost translucent, with no white spots.
Moving fertilized koi fish eggs from a pond/tank
To give eggs the best chances of safely making it into the hatching phase, you should definitely consider moving them out of the breeding aquarium and placing them in a nursery tank.
Adult koi and even other fish will sometimes eat koi fish eggs.
Even the “parent” couple will gulp them up when looking for a quick snack.
Koi fry are the most vulnerable to becoming fish food when sharing a tank with adults.
That’s why it’s recommended you try to transfer the eggs into the nursery tank before hatching occurs.
If you provided your pair of mating koi with a spawning mop/rope, it’s extremely easy to transfer the eggs, as they stick to the rope quite well.
You just move the mop/rope into the nursery tank as is.
If not, you can use a fish net to carefully collect the eggs and make the transfer.
In an aquarium, you might even be able to do it by hand.
How to tell if koi fish eggs are fertilized and viable
Fertilized and viable koi fish eggs will have a clear appearance, with a slightly brownish tinge.
You’ll be able to see tiny dots inside of them, which can look like seeds.
This is how most of your koi eggs should look like when you transfer them from the breeding tank into the nursery tank.
Healthy, clearly fertilized eggs are the most likely to produce healthy koi fry.
Not all of them will hatch, and those that remained unfertilized will quickly turn white.
What to do if koi fish eggs turn white
Unfertilized koi fish eggs, along with those that just aren’t viable due to deformities of the embryo, will quickly lose their brownish translucent appearance, turning white and opaque.
It’s not something anyone can prevent, but with female koi laying anywhere between 100,000 and 400,000 eggs on average, it’s only natural to expect a large percentage of the eggs to not reach the hatching stage.
The white color on koi fish eggs signals the presence of fungus.
You should remove all white, milky, or fuzzy-looking koi eggs from the nursery tank as soon as you see them.
The fungus from unfertilized eggs can trigger a fungal infection that will wipe-out an entire batch of koi fish eggs.
Fungus aside, decomposing fish eggs will cause a spike in ammonia level, which can be fatal to both koi eggs, and freshly-hatched fry.
Nursery tank set-up
A nursery tank, or pond, is meant to not only separate the defenseless koi fish eggs and fry from hungry adults but it’s also meant to make it easier for you to create the ideal conditions for egg development and hatching.
With fewer moving parts and only koi eggs to take care of, keeping the water quality inside the nursery tank in check is easier than it would be in a pond filled with adult fish.
Here are the tank requirements you should aim for when setting up a koi nursery tank:
| Minimum Tank Size | 100 gallons |
| pH | 7.5 ideally |
| Water Hardness (KH) | 5-8dKH |
| Temperature | 68-71°F |
| Gear | Air pump + air stone for improved oxygenation; Filtering system; If using a spawning rope, secure it near the water surface (2 inches below). |
How long does it take koi fish eggs to hatch?
Viable koi fish eggs will typically hatch 4 to 5 days after the spawning phase.
Hatching can happen anytime between 3-7 days later, but on average, you should see tiny koi fry emerging on day 4.
The fry will stay near the hatching sight for at least three days, feeding on their own egg sack.
Newly hatched koi fry are a sight to see because they make the tank look like it just exploded into life!
Koi fry will be weak and shy in their first days of life, preferring to hang out in well-planted areas near the bottom of the tank/pond.
Once their muscles develop sufficiently, they will start to explore their surroundings more.
Try not to handle newborn koi fry as they’re very delicate and easy to hurt.
You’ll still have to patiently wait before introducing the fry to other fish.
How To Take Care Of The Newborn Koi Fish Fry & Make Sure They Grow To Adulthood
Before moving koi fry into a tank or pond with other fish, you’ll need to make sure they fully develop and reach an appropriate size, where they won’t be viewed as food by their tank mates.
Koi fry are easy to take care of, mostly because they are as far from picky eaters as they can be.
Feeding koi fish fry
For approximately three days after hatching, koi fish fry will exclusively feed on their own egg, getting all the nutrients they need from that single source without requiring a feeding.
You don’t have to feed them for those three days, but you also shouldn’t feed them because the uneaten food will negatively impact the water quality inside your nursery tank.
Once your koi fry are free swimming, you should feed them specialty koi fish fry food or liquid fry food.
Their mouths are too small for regular fish food for at least a month after hatching.
When your koi fry are about four weeks old, and they start swimming to the surface of the water, you can start introducing solid food, like:
- Freeze-dried daphnia;
- Freeze-dried krill;
- Freeze-dried baby brine shrimp;
- Crushed koi fish food flakes;
- Crushed hardboiled chicken eggs.
Feed your koi fry 4 to 5 times a day to ensure proper development.
Tank conditions
Koi fish fry are not only sensitive physically in their first weeks of life, but they also have a significantly weaker immune system than an adult koi fish has.
This makes them more vulnerable to stressors such as dips in water quality, quick shifts in water temperature, the presence of fungus, parasites, diseases, etc.
Water quality should be your main focus when raising koi fry. Their bioload (amount of waste produced) will increase exponentially as they mature.
Make sure your 100-gallon nursery tank is equipped with plenty of mechanical, biological, and chemical filter media, so your filtering system can keep up with the constant influx of waste.
The water temperature inside your nursery tank, once fry hatch, should stay within a 70-75°F range, while the pH level can be maintained somewhere between 7.2-8.0.
Popular Types of Koi Fish
Koi fish varieties are usually distinguished based on three main physical traits:
- Scalation;
- Coloration;
- Patterning.
The coloration of koi fish holds symbolic value in Japanese culture:
- Black – father of the family;
- Shades of red – love, power, bravery;
- Red and orangish-red – mother of the family;
- Pink – daughter.
Other koi colors can be yellow, cream, white, black or blue.
Although there are over 100 varieties of koi fish, Gosanke koi are the most popular category of koi fish.
The term “Gosanke” actually refers to “the three families” most emblematic of Japanese Koi, in quality, excellence, and popularity.
These three koi varieties are:
- Kohaku;
- Taisho Sanshoku;
- Showa Sanshoku.
You can see a list of the most sought-after Japanese Koi varieties here:
In the hobby of fishkeeping, there are 2 main types of koi fish, each with its distinct perks:
| Domestic Koi Fish | Japanese Koi Fish |
| Western bred from mixed varieties of koi | Japanese bred from ancient varieties of koi |
| Faster growth rate | Slower growth rate |
| Smaller size – 12-15 inches on average | Larger size – up to 36 inches |
| Shorter lifespan – 15 years on average | Longer lifespan – commonly reaching 50-60 years |
| Easier to keep in an aquarium | Must be kept in a pond |
Conclusion
If you end up having to care for some koi fish eggs without much time to plan ahead, it means your koi fish got busy without giving you a heads-up.
Well, you now have the basic knowledge needed to care for koi eggs and raise viable koi fry into healthy adult koi fish, which you can either keep or sell for a pretty penny.
Koi fish breeding, as a business, isn’t an easy project, but if you’re up for an occasional challenge, go ahead and try it!
Recommended Reading:
- How Big Do Koi Fish Get? Growth Rate, Tank Conditions, And More
- How Long Do Koi Fish Live? Lifespan, Care, And More
