Japanese Rice Fish (Medaka)
Oryzias latipes
The *Oryzias latipes*, culturally revered in Japan as 'Medaka', is experiencing a worldwide explosion in popularity. Despite the appearance of a tiny killi (3.5 cm / 1.4 inches), it belongs to a separate family. The Medaka is the King of the "Mini-Pond": unlike tropical fish, it thrives outdoors at frigid temperatures (even below 5°C / 41°F). Japanese breeders have created countless and expensive color variants (Miyuki, Yang Guifei, tricolor Koi) specifically designed to be admired from above (Top-View) in lotus pots or balcony ponds.
Water Temperature
10 - 28 °C
pH Value
7 - 8
Water type
Freshwater
Tank level
Surface
Adult size
4 cm
Minimum tank volume
>= 40 L
Minimum group
>= 6
GH
6 - 15 dGH
KH
3 - 7 dKH
TDS
120 - 295 ppm
Conductivity
188 - 461 uS/cm
Species description
Geographical Origin and Habitat: Japan, China, Korea. Populates flooded rice paddies, ponds, agricultural ditches and very slow-flowing swamps with rich emergent vegetation.
Taxonomy and Morphology: Japanese Rice Fish / Medaka (Oryzias latipes). Tiny cold-water fish. Large eyes to feed on the surface, upward-facing mouth and flat back.
Social Behavior: Peaceful gregariousness. They move in loose schools constantly skimming the surface of the water. They are not shy at all, approaching immediately if you cast a shadow or touch the surface of the water.
Coloration and Sexual Dimorphism: In nature they are silver-gray, but centuries of selection (especially in Japan) have created dozens of color strains (Golden, Platinum, Miyuki - with dorsal blue reflection, and tricolors). Males with more developed and jagged anal and dorsal fins.
Care, breeding and tankmates
Aquarium Setup: Perfect for Mini-Ponds (Tubs or lotus pots outdoors) or open tanks without filters. They love very slow or still water, pots with water lilies and Ceratophyllum. A traditional aquarium with currents from a powerful filter would exhaust them.
Diet and Feeding: Insectivorous omnivores. Devourers of mosquito larvae, which is why they are put in ponds. In the tank they eat any powder, crushed flake or floating micro-pellet.
Water Quality: Indestructible (Cold/temperate water fish). They tolerate freezing water at 4°C (39°F) under winter ice and 30°C (86°F) in summer pools. They tolerate polluted waters, but prefer moderately hard water (GH 8-12).
Compatibility and Cohabitation: Never put with large, active or tropical fish. In a pond or tank without a heater (Room temp) they coexist well only with Neocaridina (although they will eat some babies) and snails. Ideal for monospecific tanks.
Aquarium Breeding: Drops of grapes. The female expels a cluster of large transparent eggs and, instead of scattering them immediately, carries it hanging from her belly for hours before rubbing against floating roots (Mop or Water Hyacinths) to deposit it.
Risks and Diseases: The Closed Tank. Like Bettas or other anabantids, if put in a deep indoor aquarium (with a heater set at a fixed 26°C / 79°F) they will age in 6 months wearing out. They must perceive the seasonal or daily thermal swing to thrive.
Geographic Origin and Distribution
Japan, China, Korea. Populates flooded rice paddies, ponds, agricultural ditches and very slow-flowing swamps with rich emergent vegetation.
Origin
Selective breeding and cultivarsEast AsiaTropical oceans and reefsCosmopolitan or introduced
Fish profile
- Temperament
- Aggressive
- They are incredibly peaceful, active, and curious fish. They do best when kept in shoals of six or more. In a group, they display lovely schooling behavior and are constantly exploring their environment.
- Diet
- Omnivore
- They are omnivorous micro-predators that eagerly accept crushed flakes or nano pellets. They relish small live foods like daphnia, grindal worms, and baby brine shrimp. They will also graze on small amounts of algae within the aquarium.
- Life expectancy
- 1-3 years
- Oryzias latipes has a lifespan of approximately 1 to 3 years in aquariums, though they can reach 4 to 5 years in well-protected outdoor ponds. Their longevity is closely linked to seasonal temperature variations, which regulate their metabolic rate. Clean water and a varied diet are key factors for their health. Being highly adaptable, they tolerate a wide range of conditions but thrive in stable, planted environments.
- Adult size
- 4 cm
- The catalog's scalar value is 4 cm. The FishBase SPECIES table (v25.04 snapshot) source reports a maximum adult measurement of 4 cm for this species, recorded as standard length (SL). The taxonomic record consulted is Oryzias latipes (v25.04); male or unsexed record. These figures are not interchangeable: the first is the profile's editorial reference, whereas the second is a published maximum under a specific measurement convention; differences among sources can reflect sex, population, life stage and measuring method.
- Sex ratio
- Group
- Males are slimmer and often have longer, more pointed dorsal and anal fins. Females are generally rounder, particularly when carrying eggs. A ratio of one male to two or three females is ideal for breeding purposes.
- Feeding frequency
- Twice daily
- Feed them tiny amounts once or twice daily. They have very small stomachs, so a little food goes a long way. Uneaten food should be removed to prevent water quality degradation.
- Bioload
- Low
- Japanese Rice Fish have an extremely small bioload. They are perfect for small, planted bowls or nano aquariums. Basic filtration or heavily planted setups are usually enough to process their waste.
- Flow
- Strong
- They naturally inhabit rice paddies and slow-moving streams, preferring very calm water. Strong currents will exhaust them and cause significant stress. Sponge filters or very gentle power filters are ideal.
- Jump risk
- Yes
- They are capable jumpers, especially when spooked. A lid is generally recommended for indoor aquariums. If kept in outdoor tubs, providing dense floating plants usually deters jumping.
- Reproduction
- Oviparous
- They are remarkably easy to breed and will often spawn daily in good conditions. Females carry clusters of eggs near their vent before depositing them on fine-leaved plants or spawning mops. To maximize fry survival, the eggs or parents should be separated.
- Compatibility & tankmates
- Species only
- They are highly peaceful and do well in species-only setups or with other very small, peaceful fish. They can coexist happily with ornamental shrimp and snails. Avoid keeping them with anything large enough to fit them in its mouth.
- Tank level
- Top
- They spend the majority of their time in the top and middle layers of the water column. They love swimming among floating plants and surface vegetation. Providing cover at the surface makes them feel safe and encourages breeding.
Scientific Parameters & Models
FishBase
Window to the official scientific data of the species
Data synced on: 06/15/2026
- Trophic level
- 3.4 ± 0.45 se
- Resilience
- Low (4.5 - 14 years)
- Generation time
- 0.3 years
- Phylogenetic uniqueness (PD50)
- 0.5
- Bayesian Length-Weight
- a = 0.00479, b = 3.13
- VulnerabilityLow vulnerability (10 / 100)
Image gallery
Licensed images linked to the species or, when marked, to the closest representative taxon.
Related species
Recommendations grouped by close relatives, companion choices, and similar care.
