Created date:2018/09/18
Update date:2020/07/02
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Ask someone who wants to keep a medaka.

・I want to keep a medaka but I don't know what and how to prepare it.
・I want to increase medaka.
・I don't know how to care for medaka.

The medaka that was swimming lively at the beginning gradually decreased, and finally disappeared. 

In fact, breeding medaka is surprisingly difficult due to its depth.

In order to prevent such a result, I wrote this article in the hope that it would be helpful as a reference after discussing the failures, so please take a look.

It's been a long time since I started keeping medaka from my interest. I would like to write some useful information so that people who start keeping medaka will not fail.

White medakas in the sun.


important point
Outdoor breeding, the way to keep it on the veranda as the title of the program is posted.
If you want to keep indoors, please refer to other people's blogs.

For those considering medaka breeding
Medaka is the best fish for anyone who wants to keep fish. The body is small, so you don't need a big container, and you can buy 4 kinds of common black medaka, black medaka, white medaka, and blue medaka that you often see in stores.

Outside, you don't have to pay for equipment filters, aeration, lighting, heaters, etc.

Forget about time when you see the swarm swimming after all.
I love how I swim coolly, especially on summer mornings and dusk.

Breeding of medaka is deep

However, although killifish are easy to buy, raising them is not that easy. How did you feel when you saw someone else's blog?

Don't you think there is a lot to do when breeding? Did you think that you had to prepare a lot of preparations and prepare them? That's right.It is also a deep and difficult fish.

Medaka is a fish with a strong vitality, but it will show its true value if properly kept. If you don't take good care of yourself, such as changing water, maintaining aquatic plants, feeding, spawning and raising fry, the killifish will die in no time.

Even if you just put it in a container and raise it by yourself, it will be alive to some extent, but it will disappear.It was a series of failures because I started breeding medaka with my ignorance just as I remembered when I was a child.


1.Preparation for medaka breeding

Prepare the following 4 items before starting breeding.

Things to prepare for medaka breeding
 Choose a medaka breeding container with a wider swimming space than one with deep water
・Aquatic plants

 Choose submerged plants such as matsumo and anacaris
 Choose Akadama soil, Arakida soil, aquatic plant culture soil
 It may be a live feed such as akamushi or commercial food such as medaka food and pleco.
・Breeding water
 Place tap water that has been pumped for about half a day, or add the neutralizing agent and remove the scalp.

A medaka breeding container. Choose one with more swimming space than one with deep water. The container does not need water lily pots, planters, and bottom holes for flower pots.

 Since it breeds, it is necessary to have a container for raising the medaka hatched from eggs, in addition to the container for raising the parent medaka.

 The approximate number of individuals suitable for a container is 10 at 30 cm, 20 at 60 cm, and 30 at 90 cm. The larger the container, the greater the amount of water, and the more stable the water temperature and water quality, the lower the risk of medaka having poor body length and death.

Water lilies are available in pottery or FRP, a light fiber reinforced plastic. The plastic and toro vessels used by plasterers are also wide and easy to grow.If you don't care about durability, you can use styrene foam.

The largest container in my home.


Aquatic plants
It will be a place for spawning and hideouts for medaka. It prevents medaka spawning eggs and hatched fry from being eaten by other medaka fish.

 It is also useful for water purification, and aquatic plants clean up harmful substances such as food residues and feces.

Submerged plants such as matsumo and squirrels, and the floating plant Hotei grass for spawning beds, are aquatic plants that go well with medaka. Matsumo and Anacaris are perennials that can also be wintered, and Hotei grass is a severe annual winter.

 You can purchase Matsumo and Anakaris from around 600 yen, and Hotei grass from around 300 yen.

※Hotei grass can be wintered if it is grown indoors in a warm and sunny environment.

Thin out aquatic plants
 The growth of Matsumo and Anakaris is fast, and it is good for growing underwater, but when it grows on the surface of the water, it gradually takes away the swimming space of medaka, weakens due to stress, and dies.

Be sure to thin out aquatic plants. If you observe every day and feel that the space for swimming killifish is running out, you can skip it.

If you do it tomorrow or on the weekend, it will take away the swimming space of the medaka in no time. 

Story of failure
 I think it's best to plant what you like and what you want to grow.
 It's fun to grow big day by day with medaka, and you can also enjoy it if you are a kind of flowering aquatic plant.

 However, if you plant a lot of this, the medaka swimming space will eventually decrease and you will die.

Also, when planted directly, the soil becomes rooted and the soil rises and there is no space for the medaka to swim and it will die.直植えをした場合は用土が根だら

Matsumo. A medaka kid is swimming.


Akatamachi is colorful and beautiful. Another advantage is that there is little turbidity and it is easy to observe aquatic life. However, aquatic plants also grow, but they do not grow well because they have no nutrition.
 There are large, medium, and small grains, but I think that you may like them. The shape of the ball will collapse in a year.
 Aquatic plant culture soil is also an option. Because of its high nutritional value, planting aquatic plants will make it grow bigger. However, because of nutrition, moss and algae are generated.
 Arakida soil is the soil used in rice fields. Medaka is a fish that has been familiar to us since ancient times as a rice field fish.よって Therefore, using the medium close to the rice field means that the medaka fish are raised in an environment close to the rice field.
 The advantage is that gravel and stones can be maintained and reused every year. However, the problem is that the bacteria that purify the water quality do not settle.

You can buy Arakida soil and Akadama soil for about 2L 300 yen.

Is Arakida soil good for the soil?
 I would like to recommend Arakida soil as the medium. After all, the place where medaka lives is in the still waters such as rice fields and streams that flow slowly on the sides of the rice fields.
 Since food such as microorganisms and plankton is also generated in this soil, the medaka will be supplied with abundant food.

 However, because the mud floats up, it is not suitable for ornamental use, and there is nutrient in the Arakida soil, which can lead to eutrophication of water and moss and other algae overgrowth.

 In the rice fields, rice absorbs the nutrients, but if you look closely, you can see that water-blooms and moss are everywhere. Also, it does not purify water as cleanly as a river or pond.

 Still, I would like to recommend Arakida soil.

 After all, the mortality rate of medaka is low, and the incidence rate of the typical medaka disease "slimming disease" is suppressed. There is no problem with the mud rising if you stack red soil.

Arakida soil has some effects and some side effects, but I think it might be worth considering.

Should the soil be put in the first place?
 It is said that the use of soil does not work like natural filtration, and bacteria that settle in the soil are purified, but it is unnecessary because nutrients in the soil itself deteriorate water quality rather than purification.

 If you want to grow in clean, clear water, you can use gravel or stone.

 However, it also has the drawback that feces and dust accumulate and the bacteria that settle in the soil are few and are not easily decomposed.

 There is also a bare tank called a bottom tank where no soil is used on the bottom floor and the maintenance is prioritized.

 However, it has the drawback that stools and debris will accumulate, and that there will be no bacteria that settle in the soil and that it will not be decomposed.

I don't think there is an answer which is better.

I tried various kinds of medaka food, but now I am using medaka food and pleco. Pleco especially eats not only medaka but also other aquatic organisms such as southern shrimp and Himetanishi. In particular, it supplements the lack of moss, which is the food of Himetanishi, and the survival rate of Himetanishi is very high.


キョーリン ひかりクレスト プレコ 300g えさ 関東当日便
価格:1225円(税込、送料別) (2020/5/29時点)

Breeding water
 Add tap water that has been pumped for about half a day or one that has been neutralized and decalcified.

If you add a neutralizer, the bleach will come off immediately, but due to the difference in water temperature, it is not recommended to add medaka immediately afterwards. Let's put the killifish in about half a day.

 Bacteria break down dirt and clean the water, but initially there are no bacteria.

 If you feed medaka in this state, the water will be contaminated with leftover food and feces, and ammonia will increase the number of bad bacteria because biological filtration will not be in time.

 With such a mechanism, the water may become cloudy in white.

It is necessary to control feeding as much as possible, and in severe cases it is necessary to change the water, but even if it is completely changed, it will only return to a bacteria-free environment, so wait for the growth of good bacteria.


2.Kind of medaka

There are more than 500 kinds of medaka. It is no exaggeration to say that new breeds are being created every day through breeding. The rare varieties that are very valuable for appreciation, from pin to sharpness, lightly cost more than 10,000 yen per animal, and some species cost 1 million yen. If you are not particular about it, you can purchase it cheaply by choosing from the following four standard types.

Kind of medaka

・Black medaka

It is a medaka that is close to the original species that inhabit all over Japan. There are two main types of medaka, Kitano medaka in northern Japan and Minami medaka in southern Japan. Black medaka is an improved variety. However, wild medaka is designated as an endangered species, and I think most of the alien species, Kadashi, can be collected in rivers and ponds in the city.
It is an exotic species native to North America. It looks like a black medaka. It looks like a black medaka. However, since the female grows up to about 6 cm, you will understand when you catch it. Kabayashi invades and lives after the medaka, which has been displaced due to environmental pollution. It is designated as an alien alien species and has penal regulations, and if violated, individuals are imprisoned for up to 3 years, or fines up to 3 million yen, and corporations up to 100 million yen. You cannot take it home or raise it.
・Japanese medaka
Speaking of medaka, I think many people think of medaka. It is a medaka that has obtained citizenship from the black medaka.
・White medaka
It is a beautiful medaka that looks best when kept in a container. However, it is a breed that is difficult to breed because it is genetically weak compared to black medaka and medaka.
・Blue medaka
Blue medaka is difficult to distinguish from white medaka at first glance, but if you look closely, you can distinguish it because it is bluish. Like white medaka, it is a genetically weak variety that is difficult to breed.


価格:640円(税込、送料別) (2020/5/29時点)


価格:600円(税込、送料別) (2020/5/29時点)


価格:600円(税込、送料別) (2020/5/29時点)


価格:640円(税込、送料別) (2020/5/29時点)

Which order has the highest vitality?
The order in which the vitality is strong among the four types is as follows.

Black medaka > Japanese medaka > White medaka > Blue medaka

The medakas swimming in the green water.


3.Medaka breeding environment

It is an important breeding environment for breeding medaka. If the breeding environment is poor, the durable medaka will not grow, and on the contrary, it will become sick and die.

Important to the environment of medaka
 Nikko, this is the most important. Medaka produces vitamin D by exposing it to sunlight to form strong bones and muscles. Also, if it is not exposed to sunlight for about 12 hours a day, it will not spawn.
The breeding water can be tap water. However, since chlorine is contained in tap water, which is a harmful substance for aquatic organisms such as medaka, descaling is required.
 This is the most important breeding environment for killifish. The strong medaka grows in the sunlight. It is said that the sunlit environment is said to be best in the morning and gradually becomes shaded in the afternoon.
 My house is in the opposite environment, in the shade in the morning, and the sun starts to hit from around noon and the sun is intense.

The worst daylight environment is in direct sunlight all day or in shade all day. However, be careful of direct sunlight in the summer.
 The medaka may die due to boiling water in the container. In this case, move the container to the shade, make a shade and make a shade.
Breeding water
Medaka is a fish that lives in the uncontrolled waters of rice fields, streams, ponds and swamps. The larger the amount of water, the better, but it is said that 1L per animal is a standard.
 Water temperatures can range from 5 to 28 degrees Celsius, slow down above 30 degrees Celsius, and even die if the water temperature is too high. On the other hand, if the temperature drops below 5°C, you will not eat any food and you will go into hibernation.
 Aiming for green water that is rich in phytoplankton is a shortcut to make medaka live longer.



4.Medaka life cycle

It is a lifelong activity of medaka. There is no loss to remember when breeding.

Let's Know About Medaka

・Life of Medaka
 It has a short life of 1 to 2 years.

・1 year of medaka

March         I wake up from hibernation and eat food
      April          Start spawning
      June            Spawning becomes vigorous
   October        Finish spawning
      December Hibernate
・Spawning medaka
Spawning starts when the temperature rises above 15℃. Medaka will incubate at the time of spawning.
 Spawning is possible when the daylight hours of the day are about 12 hours. The number of eggs laid at one time is around 10, but there are individual differences.
Spawning starts when the temperature rises above 15℃. Medaka will incubate at the time of spawning. Spawning is possible when the daylight hours of the day are about 12 hours.
 The number of eggs laid at one time is around 10, but there are individual differences. For spawning, prepare a dedicated spawning floor or float water plants such as hotei grass to spawn. When the egg is laid, the parent medaka eats it, so isolate it.
 Either put the spawning bed or aquatic plants in a separate container, or pinch each spawned egg with your finger and immerse it at the bottom of the container before hatching. Even if you pinch it with your fingers, fertilized eggs will not crush, so there is no problem.
 It is okay if there is tap water in the isolation container. Since the chlorination of tap water has the effect of sterilizing the mold fungi adhering to medaka eggs, it is advantageous to use tap water as it is.
 However, it is only the first time to put tap water into the isolation container. Eggs hatch at a cumulative temperature of 250°C. If it is 25℃, it will be calculated in 10 days.

If you neglect to isolate the eggs
When medaka eggs are isolated, they are eaten almost before hatching. Even if you survive and hide in aquatic plants such as Matsumo and Anacaris, you should think that you can hardly survive.
This is because in the narrow world of containers, they can be eaten without escape from the parent medaka. I also thought that it would naturally increase, so I broke the eggs rather than isolating them, and as a natural result, the number of medaka did not increase at all.


It is a white medaka holding an egg.

Medaka eggs laid on the spawning bed.

A juvenile medaka fish that has just hatched.


5.Medaka rearing cycle

Breeding of medaka fish will take place from March to November.

March Replace the water in the breeding container,
                 Replanting aquatic plants,
                 Start feeding※1
 April    Isolation of laid eggs,
         Caring for fry(~October)
    May    Maintenance of too many aquatic plants(~October)
 July    Take measures against heat(~September)
   October Removal of dead aquatic plants,
                   Put parent killifish and child killifish in the same container
   November Finish feeding
   December~February next year do nothing※2
 Medaka is fed about 3 times a week in the morning or evening.
 Do not give it when it is hot during the day as it will cause you to feel unwell.

It is especially dry during the winter, so please add more water.

6.Medaka disease(Illness to lose weight)

If you keep medaka, you will always be worried about "slimming disease".「 "Slimming disease" is not starvation, but it is a disease that causes thinness and death at the end even if feeding is continued. There are various opinions, but the cause is not clear. Although it is such a "thinness disease", it can be prevented to some extent.

Prevents slimming disease

Those who find it difficult to keep medaka fish!

How to keep a medaka for someone busy!

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