Medaka (Rice Fish) lives long if you know the cause of death

Medaka (Rice Fish) lives long if you know the cause of death


Medaka is essentially a very strong fish, and it survives well even if you leave them. If you buy medaka and die immediately, it is possible that you are using tap water, not removing chlorine, and not combining water. If the medaka you have kept for a while dies, conceivable the water quality might deteriorate or it might be ill.

You did not remove chlorine

Tap water contains chlorine that has bactericidal action, and if used as it is, medaka will die easily. If you use tap water, put it in a bucket and leave it for half a day before using it, or use a commercially available chlorine remover to remove chlorine.

Combining water

If you buy medaka, you will need to combine the water containing the medaka with the water you made in the tank, instead of putting it in the tank immediately. Suddenly, if you put it in a tank, the sudden change in temperature and water quality will weaken the medaka.

First, float the container (or bag) from which you bought the medaka in a water tank and wait for about 30 minutes. Now that the water temperature is about to match, then put about a glass of water in the tank into a container with medaka. Wait for another 30 minutes and put about a glass of water in the tank into the container with medaka again. Repeat this three times to match the water quality. Then transfer the medaka to the tank.

The above 2 points are described in detail in "The first thing to do if you buy Medaka", so please read this article first.

Deterioration of water quality

In an environment where water is not replaced, the water quality will only deteriorate due to the excrement of medaka fish and the leftover food. Excrement contains ammonia and is toxic to fish and all living things. Also, the leftover food may rot and give off toxic substances.

These toxic substances are broken down by microorganisms in the water and become less toxic. In other words, if there are microorganisms in the water, the rate of deterioration of the water quality will be reduced.

The most effective way to increase microorganisms in water is to spread low sand. By putting sand, gravel and soil on the bottom of the water, microbes will grow there.

Microorganisms come from various places, but in the case of water made from tap water in a completely new water tank, there are few microorganisms. Microorganisms increase with the passage of time, but if you keep many medakas in an environment where there are still few microorganisms, the excrement will not be decomposed and the water quality will deteriorate rapidly.

If it is a new tank, please change the water frequently. Change about 1/3 of the water once every 2-3 days. The water becomes stable in about a month, so change about 1/3 the water once a week thereafter. In addition, there is no need to change water in winter.

The water quality is very stable when there are aquatic plants in the tank. In the Medaka's House, in the water tank where the aquatic plants have become like carpets, we do not changed the water almost, but only add the amount reduced by evaporation, but the medaka is doing well.


Medaka is inherently strong and rarely be ill. What gets sick is when the water quality is bad and the medaka is unhealthy, or when the water temperature changes rapidly and the medaka's strength is falling. Diseases can be treated at an early stage, but because they are so small, the chances of recovery are low. Let's stabilize the water quality and the water temperature so as not to get sick.

If the medaka died of illness, immediately remove the corpse from the tank and change about 2/3 the water. The disease may be transmitted to other medaka. Match the water temperature so that there is no change in water temperature. You also need a chlorine remover to remove the chlorine.

Lack of oxygen

If the area of ​​the water surface is small or the number of medaka is too large for the amount of water, medaka may die due to lack of oxygen. As a omen, Medaka is raising noses and breathing is extremely intense.

If there are such signs, reduce the number of medaka in the tank or change to a tank with a large surface area.


If you keep different sizes of medaka together, the large medaka will chase after the small medaka, and the small medaka's stress may increase and cause illness. By change the tank bigger or creating a hiding place, you can always keep it from being chased.

In addition, as there is compatibility with medaka, let's transfer the medaka always chased to another water tank.

Water flow

If you are doing aeration, a large stream of water will be created in the water tank, and the medaka will weaken and die because they will always swim against the stream. Medaka is a fish originally living in the paddy field. Let's not make the water flow in the tank. Aeration is not essential for Medaka.

Medaka is sometimes die by the roots of water hyacinth

Many people maybe float water hyacinth in the medaka tank or water lily pot. Water hyacinth is strong, grows only by floating, and blooms and blooms, but in fact, the root of water hyacinth may kill medaka.

Water hyacinth grows and grows steadily as it is sunny. The roots grow and spread, and the Medaka space tends to disappear. Because of the roots of the water hyacinth, the medaka can often become immobile and die.

If you use water hyacinth, be sure to cut the roots from time to time. Even if the roots are cut in half to about 2/3, the water hyacinth does not wither.

Water plants that are unexpected pitfalls

Aquatic plants supply oxygen to water by photosynthesis but do not perform photosynthesis at night when there is no light. Because aquatic plants live and breathe, they constantly consume oxygen and exhale carbon dioxide.

There are too many aquatic plants and you may run out of oxygen in the water during nighttime photosynthesis. If a lot of medaka die at the water grass tank at once, observe the condition of the killifish at night when the light is not illuminated.

If medaka only raises their nose at night, there is a high possibility that there are too many aquatic plants.

You are buying medaka grown in a greenhouse

Recently, many medaka fish cultivated in greenhouses and indoor facilities like factories are on sale. The reason is that it can be bred even in winter and that it can increase the survival rate of fry and produce a lot.
However, medaka fish grown in such an environment often cannot adapt to the environment of the aquarium of ordinary households. Medaka is a highly adaptable fish that has adapted to the harsh environment of Japan, which has four seasons, but it is the result of the selection of weak individuals that would otherwise die in nature.

Among the medaka fish grown in greenhouses, there are many medaka fish with extremely low adaptability to the environment. Adapting it to a non-greenhouse environment is extremely difficult. As a result, the same breeding method as tropical fish is required.
Medaka is said to be an easy-to-breed fish because it has lived in Japan for a long time and has adapted to the Japanese environment, so it can be easily bred in Japanese homes.
In other words, it is originally a fish that does not require advanced breeding skills like tropical fish, but only medaka fish raised in greenhouses have become extremely difficult to breed.


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