Of all the Japanese Rice Fish colorations, a school of Moonlight Medakas is one of the most eye-catching attractions in the aquarium!

The Japanese Rice Fish Oryzias latipes have been popular aquarium fish for many years. They have been kept in the aquarium since the 17th century in Japan. They are ‘amphidromous’ meaning they are fish that move between fresh and salt water during some part of their life cycle. They are known by several common names including Japanese Medaka, Japanese killifish, Medaka, Rice Fish, and Ricefish.

The wild version of fhe Japanese Rice Fish is not particularly attractive, but fortunately for the aquarist it comes in many colors. There are a few selectively bred colors varieties like the Gold Medaka and more recently, the Moonlight Medaka. These colorful fish are quite attractive and well worth keeping in the home aquarium.

The best known version of the Japanese Medaka is the Gold Medaka (gold color morph), which has been around for hundreds of years. The Moonlight Medaka is a shiny silver color morph that is a relatively new addition to the aquarium hobby. These are genetically enhanced versions of this fish that contain a sea jelly gene that causes them to glow under a black light. The Moonlight Medaka is a very sturdy fish that can live in warm or cold water. This cold water tolerance also makes it a good choice as a pond fish.

Scientific Classification


Japanese Rice Fish – Quick Aquarium Care

Aquarist Experience Level:Beginner
Aquarium Hardiness:Very Hardy
Minimum Tank Size:10 gal (38 L)
Size of fish – inches1.6 inches (4.06 cm)
Temperature:64.0 to 75.0° F (17.8 to 23.9&deg C)

Habitat: Distribution / Background

The Japanese Rice Fish Oryzias latipes was described by Temminck and Schlegel in 1846. They are naturally found in Japan, Korea, China and Viet Nam where they inhabit slow moving waters. They have also been introduced in Iran and Turkmenistan. Other common names they are known by are Japanese Medaka, Japanese killifish, Medaka, Rice Fish, and Ricefish. The Japanese Rice Fish is not listed on the IUCN red list.

They are normally found amoung the dense rice paddies. It is common for these fish to travel between fresh and saltwater environments and can be found in the ocean as well as freshwater rives. There are no wild populations of the Moonlight Medaka.

The Gold Medaka and the Moonlight Medaka are color morphs of the Japanese Rice fish, and are not found in the wild. The Japanese Rice Fish or Medaka are used extensively for scientific research. This fish has actually gone into space on the space shuttle Columbia in 1994 and was the first vertebrate to mate in orbit.

  • Scientific Name: Oryzias latipes
  • Social Grouping: Groups – These fish normal stay in a loosely schooling groups.
  • IUCN Red List: NE – Not Evaluated or not listed


The body of the Japanese Rice Fish is slender and elongated with a slightly arched back. This fish will reach 1.6 inches (4 cm) and it has a life span of about 4 years. The wild form of this fish is not particularly flashy, being a rather bland cream color but with some iridescent blue-green flecks. It does have beautiful large almost luminescent eyes. The wild strain is seldom seen in the aquarium trade.

The Moonlight Medaka and the Gold Medaka are captive bred color morphs. They are fluorescent ornamental varieties, genetically modified fish that glow under a black light. They were developed by attaching a product of fluorescent protein extracted from jellyfish into embryonic Japanese Rice Fish. The Gold Medaka has been around for hundreds of years, while the Moonlight Medaka is a relatively recent development.

  • Size of fish – inches: 1.6 inches (4.06 cm)
  • Lifespan: 4 years

Fish Keeping Difficulty

These fish are great fish for beginners. They are extremely peaceful and can be easily kept in a small unheated aquarium. They are not messy fish so tank maintainance is easy to do. These fish are easy to feed and will take most anything. They are easy and interesting to breed in small aquariums as well.

They are not very demanding fish and can tolerate a variety of conditions. They are often kept in outdoor ponds year round and there are reports that they can survive when the ponds ice over.

  • Aquarium Hardiness: Very Hardy
  • Aquarist Experience Level: Beginner

Foods and Feeding

The Japanese medaka primarily omnivores. They will eat all types of foods, including flake and freeze-dried foods.They will also eat smaller prepared or frozen foods, so feeding is not a problem.

  • Diet Type: Omnivore
  • Flake Food: Yes
  • Tablet / Pellet: Yes
  • Live foods (fishes, shrimps, worms): Half of Diet
  • Vegetable Food: Some of Diet
  • Meaty Food: Most of Diet
  • Feeding Frequency: Several feedings per day

Aquarium Care

Caring for this fish is pretty simple. Not much different then caring for a goldfish! They are very tolerant of cold temperatures, infact prefer these conditions. So with the use of a high quality filter and regular water changes this fish is very easy to care for. Do regular weekly water changes of 30%.

  • Water Changes: Weekly – Water change should be about 30% weekly.

Aquarium Setup

Japanese Rice Fish will spend their time in all parts of the aquarium. These fish will do well in freshwater but prefer brackish waters. The tank does not have to be large, with 10 gallons being fine when kept singly. A tank size of 20 gallons or larger is recommended if you plan on having a school of this species. The Ricefish actually prefer an unheated environment. A high quality filter is recommended as with most fish. Provide good filtration and do regular water changes. A tight fitting lid is a necessity because this is a fish that likes to jump.

These fish live in fields of dense rice paddies and this environment should be replicated having dense floating vegetation. As with many fish, the lampeyes will do best and are most effectively displayed in tanks which simulate their natural habitat. The substrate can be small gravel or sand. A well lit planted aquarium with a dark substrate will showcase this fish best. Some driftwood will make nice areas for the fish to hide when nervous.

  • Minimum Tank Size: 10 gal (38 L) – A 20 gallon tank or larger is recommended if you plan on keeping a school.
  • Suitable for Nano Tank: Yes
  • Substrate Type: Small Gravel
  • Lighting Needs: Low – subdued lighting
  • Temperature: 64.0 to 75.0° F (17.8 to 23.9&deg C)
  • Range ph: 7.0-8.0
  • Hardness Range: 9 – 19 dGH
  • Brackish: Sometimes – These fish will travel to the oceans and back into freshwater rivers and marshes. They do however prefer brackish waters but will do just fine in a freshwater environment.
  • Water Movement: Moderate
  • Water Region: All – The Medaka will swim in most parts of the aquarium.

Social Behaviors

The Japanese killifish are very peaceful, great for a community tank with other peaceful fish of similar size. These fish are so small, and when schooling they do not stay in tight groupings. So be cautious when housing them with larger fish, even peaceful ones, as they may see this little fish as an easy meal.

  • Venomous: No
  • Temperament: Peaceful
  • Compatible with:
    • Same species – conspecifics: Yes
    • Peaceful fish (): Safe – Medaka are very small and school loosely, so larger fish may see them as food.
    • Semi-Aggressive (): Threat
    • Aggressive (): Threat
    • Large Semi-Aggressive (): Threat
    • Large Aggressive, Predatory (): Threat
    • Safe
    • Shrimps, Crabs, Snails: Safe – not aggressive
    • Plants: Safe

Sexual differences

Sex is not always easy to determine but males tends to be more slender. The dorsal and anal fins of the males are also larger than those of the females.

Breeding / Reproduction

The Medaka can be placed in a breeding tank with fine leaved plants or a spawning mop. They can be introduced to the tank as a pair or in a small school. Spawning usually occurs at dawn. After the eggs are fertilized by the male, many of them will remain attached to the females belly. As she swims around and comes into contact with water plants, the eggs will detach from her belly and will be attached to the plants. The eggs will hatch in 10 to 12 days. The newly hatched fry should be fed infusoria, a liquid fry food, or newly hatched baby brine.

  • Ease of Breeding: Easy

Fish Diseases

Japanese Ricefish are extremely hardy and disease is not usually a problem in a well maintained aquarium. That being said there is no guarantee that you won’t have to deal with health problems or disease. Anything you add to your tank can bring disease to your tank. Not only other fish but plants, substrate, and decorations can harbor bacteria. Take great care and make sure to properly clean or quaranteen anything that you add to an established tank so not to upset the balance.

A good thing about Japanese Ricefish is that due to their resilience, an outbreak of disease can often be limited to just one or a few fishes if you deal with it at an early stage. When keeping more sensitive types of fish, it is common for all fishes to be infected even before the first warning signs can be noticed. The best way to proactively prevent disease is to give your Japanese Ricefish the proper environment and give them a well balanced diet. The closer to their natural habitat the less stress the fish will have, making them healthier and happy. I stressed fish will is more likely to acquire disease.

Japanese Ricefish are very resiliant. Yet it is recommended to read up on the common tank diseases. Knowing the signs and catching and treating them early makes a huge difference. For information about freshwater fish diseases and illnesses, see Aquarium Fish Diseases and Treatments.


Though the Japanese Rice Fish or Japanese Medaka are commonly available and inexpensive, genetically altered versions of the Japanese Rice Fish are not legal in some of the U.S. states.



 Nihonmedaka (Image Credit: NOZO, Wikimedia Commons CC BY-SA 3.0 Unported)