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Care guide · Freshwater · fish

Medaka

Oryzias latipes

Medaka (Oryzias latipes)
Photo: Seotaro / Estrilda · Source · CC BY-SA 3.0Resized and compressed; no cropping or colour changes.

The medaka (Oryzias latipes) is a small Japanese ricefish often seen swimming near the surface. Wild-coloured fish are relatively understated, whereas domestic strains may display silver, orange or strongly reflective patterns. The species is interesting for a quiet, relatively cool aquarium, but not automatically for every warm community tank. Its unusual reproductive behaviour is particularly noticeable: after fertilisation a female may temporarily carry a cluster of eggs beneath her body before depositing them among plants. This is different from giving birth to live young. Plan a suitable group before buying medaka, monitor temperature and ensure that even these small fish can actually reach and swallow their food.

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At a glance

Temperature
18–22 °C
pH
7.0–8.0
GH
9–19 °dGH
Aquarium length
At least 45 cm
Adult size
Approximately 3.6–4 cm
Group size
At least 8 individuals
Temperament
Peaceful

About this species

Oryzias latipes belongs to the Adrianichthyidae. Its scientific name matters because medaka is used more broadly as a trade name. The northern Japanese ricefish Oryzias sakaizumii is a separate species, and older distribution records may also include other related ricefishes. Names such as Miyuki or Yokihi describe breeding lines rather than new biological species. An expensive colour strain needs the same attention to water quality and space as a wild-coloured fish.

The consulted records give approximately 3.6–4 cm standard length, excluding the tail fin. Males may have larger dorsal and anal fins; body and fin shape help distinguish the sexes, although a photograph of an immature fish is not always conclusive. The upward-facing mouth suits feeding near the surface. Our photograph shows a wild-coloured specimen of documented origin from Hamamatsu, Japan. Not every reflective or translucent aquarium strain looks identical, and colour alone does not reliably identify the species.

Natural habitat

This guide concerns the southern Japanese medaka. Older sources list a much larger Asian distribution; subsequent taxonomic divisions mean that list cannot be repeated without qualification. Shallow, quiet waters, irrigation channels and rice-field environments explain the usefulness of open surfaces and marginal vegetation. Such habitats change with season and weather. That does not mean every captive-bred individual can tolerate all outdoor temperatures without preparation.

In an aquarium, fine plants, shelter and an open surface are practical features. Cryptocoryne can fill the margins without requiring strong water movement. A summer outdoor container differs from a stable indoor aquarium: small dark containers can heat rapidly in sunlight, while rain alters water composition and depth. Prevent escape through overflows and never move fish or plants into natural waters. This guide does not guarantee safe overwintering beneath ice or treat domestic strains as automatically winter-hardy.

Aquarium and layout

A footprint of at least 45 × 30 cm is the consulted lower limit for a dedicated group. Do not interpret it as permission for a crowded community. Additional species require more swimming space, separate feeding opportunities and a fresh assessment of the entire population. Height and litres alone are less informative for this small surface fish than genuinely open horizontal space.

Plant the margins and leave an open area where food remains visible. A gently operating, established filter processes waste without continually driving the fish away from the surface. Protect its intake whenever fry may be present. Close escape gaps and check cable openings as well. A broad feeding area is more useful than a tiny opening in a dense carpet of floating plants. Arrange the aquarium so fish can move away during short pursuits and so you can check daily that every individual is eating.

Water conditions

Our conservative starting point for routine indoor care is 18–22 °C, pH 7–8 and GH 9–19. Sources give wider ranges, and laboratories also keep particular strains at higher temperatures. These represent different purposes and conditions: a research temperature for controlled reproduction is not a compulsory year-round recommendation for a home aquarium. Ask about the purchased strain’s previous conditions and avoid sudden changes.

Monitor an unheated aquarium with a thermometer; unheated does not automatically mean sufficiently cool during a heatwave. In warm conditions pay additional attention to gas exchange and water quality, without suddenly adding ice-cold water. Measure ammonia/ammonium and nitrite and perform appropriate partial water changes. Do not routinely add salt merely because the species tolerates it: ordinary freshwater breeding lines do not automatically require brackish water. No salt dose or KH target is supplied without adequate strain-specific evidence.

Behaviour and group size

Keep at least eight medaka together. They are generally peaceful group fish, but males may briefly defend a spot and chase others during reproduction. Provide breaks in the line of sight and check that a female is not being pursued continuously. Mixing colour forms does not create greater biological diversity; breeding may also mix carefully selected characteristics between lines.

A quiet species aquarium is often easier to manage than a busy community. We do not recommend combining medaka with a Siamese fighting fish: the latter’s existing care profile requires warmer water, and both fish use much of the upper zone. A shared pH alone does not remove these differences. Keep related Oryzias species separate when preserving species or breeding lines matters. Peaceful behaviour toward adult fish does not exclude predation on tiny fry or shrimp. A separate, closely monitored space is often the safer choice for successful rearing.

Feeding

Medaka take small animal prey and suitable dry foods. Use a complete food with sufficiently small particles as the staple and vary the diet with appropriate small live or frozen prey. A large flake may remain on the surface without being easy to eat. Crumble food when necessary and check that it is swallowed. Feed in a quiet surface area so food does not immediately disappear into the filter.

The linked complete Supervit flakes and supplementary dried daphnia serve different purposes. Daphnia alone does not provide a complete diet. Offer small portions and watch whether less dominant fish also catch food. Adjust amounts to appetite, temperature and body condition; a laboratory schedule with continuously available food is not a household rule for a small tank. Remove leftovers before they decay between plants. Adult products are not automatically suitable for newly hatched fry: starter food must be assessed separately through observed ingestion.

Breeding

After fertilisation, the female often temporarily carries a visible cluster of adhesive eggs. These are subsequently attached to fine plant material or an appropriate spawning mop. Do not pull at an egg-carrying female to remove her eggs. For deliberate breeding, inspect the spawning material carefully and move eggs to a separate rearing area containing matching water. Keep different lines apart and record the spawning date so different ages are not mixed unknowingly.

Prepare fine starter food and observe whether fry actually feed. Institutional protocols use foods including Paramecium and fine dry food, followed by suitable Artemia; other methods introduce very young Artemia earlier. Food size and observed ingestion determine the choice, not a calendar date alone. Larger juveniles can eat much smaller ones. Separate substantially different sizes, protect the filter intake and keep the surface free of pollution. A few spontaneously surviving fry do not prove that all young fish receive enough food in a planted community aquarium.

Sources & review

This care guide draws on several specialist sources and was last checked on .

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