Japanese Rice Fish (Medaka)

Geographic Origin and Biotope: 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.

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 Tankmates: 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 Reproduction: 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.