Medaka – The Japanese rice fish

Medaka – The Japanese rice fish: A comprehensive guide to keeping, breeding and the impressive biodiversity

The medaka (Oryzias latipes), also known as the Japanese rice fish, has evolved from an inconspicuous inhabitant of Asian rice fields to one of the most fascinating ornamental fish for aquariums and ponds. Its extraordinary robustness, simple care and breathtaking variety of colours and shapes make it an ideal fish for beginners. Not only is it an easy-to-care-for pet, but it also offers a window into the rich Japanese culture. This extended blog post provides you with an in-depth look into the world of medaka and all the information you need to successfully keep and breed these enchanting fish.

What is a medaka? Origin, culture and biological characteristics

The name medaka (メダカ) is derived from "high eyes", which aptly describes its upward-facing eyes – an adaptation to its preferred lifestyle at the water's surface. Originally native to East Asia, medaka were a common fish found throughout Japan, Korea and parts of China. Due to their close association with traditional rice paddies, they are deeply rooted in Japanese culture and symbolise simplicity and peaceful nature.

Biologically, the medaka belongs to the rice fish family (Adrianichthyidae) and is a model organism in genetic research, particularly because of its transparent larvae and ability to survive even under extreme conditions. Its tolerance to a wide range of temperatures (close to 0 °C to over 30 °C) and water values makes it one of the most resilient freshwater fish.

Optimal keeping of medaka: pond or aquarium?

Medaka are incredibly adaptable and thrive in a variety of environments. The choice between an aquarium and a pond depends on your personal preferences and space.

The medaka aquarium: a microcosm in your living room

Keeping medaka in an aquarium allows you to observe the fish up close and admire their fine details.

• Tank size and stocking: For a healthy group of 10 to 15 medaka (a small shoal), a tank of 80 litres or more is ideal. Medaka are very sociable fish, so you should never keep them alone.
• Water temperature: Medaka do not need heating. A stable room temperature between 15 and 28 °C is perfectly adequate. They are hardly bothered by temperature fluctuations throughout the day.
• Water values: They are very tolerant. A pH value between 6.5 and 8.0 and a water hardness (GH) of 5 to 20 dGH are optimal, but not mandatory guidelines. It is more important to change part of the water regularly to maintain high water quality.

Tank setup

• Substrate: Dark gravel or sand makes the bright colours of the fish appear more intense, especially those of the shiny "Miyuki" or "Hikari" breeds.
• Plants: Dense planting is essential. Fine-leaved plants such as hornwort (Ceratophyllum demersum) or waterweed (Egeria densa) serve as perfect hiding places and spawning grounds. Floating plants such as duckweed or water hyacinth are also important as they dim the light and create a natural environment.
• Filtration: A small sponge filter or an HMF (Hamburg mat filter) is sufficient. Medaka prefer a calm water surface.

The medaka pond: natural beauty outdoors

Medaka are the perfect candidates for small garden ponds.

• Requirements for the pond: A container of at least 80 litres is recommended. For overwintering outdoors in Central Europe, the water depth should be at least 50 cm so that the pond does not freeze completely.
• Location: The pond should not be in direct sunlight all day to prevent overheating. A location with partial shade or morning and evening sun is ideal.
• Plant selection: Water lilies, water hyacinths, water lettuce and cattails not only offer visual appeal, but also important protection and spawning opportunities.
• Overwintering: Medaka can survive the winter outdoors. As soon as the water temperature falls below 10 °C, they become inactive and enter a kind of cold torpor. They will survive as long as the pond has an ice-free area. You can also overwinter them in a cool, frost-free room (cellar, garage).

Nutrition and feeding: A varied diet

Medaka are uncomplicated omnivores. However, their diet should be varied to promote their health and the intensity of their colours.

You can find Hikari's Medaka Premium fish food in our online shop.

The interesting breeding of medaka

Breeding medaka is not only easy, but also a highlight of keeping them.

• Sexual differences: Distinguishing between the sexes is relatively easy if you look closely. Males often have a deeply indented anal fin and a longer dorsal fin that looks like a fin with ‘fringes’. Females have a rounded dorsal fin and carry a clear, grape-shaped egg ball on their genital papilla during the spawning season.
• Spawning: Medaka spawn almost daily in the morning hours as soon as the water temperature exceeds 15 °C and there is sufficient light. The females swim around with the egg sac and rub it against finely feathered plants or special spawning brushes made of yarn.
• Rearing the larvae: Depending on the temperature, the eggs take 10 to 14 days to hatch. To maximise the survival rate, you should carefully collect the eggs and transfer them to a separate rearing tank with good, unpolluted water. The hatched larvae are tiny and initially require infusoria or dust food for the first few days before they can switch to micro worms or freshly hatched Artemia nauplii.