Youkihi Orange Medaka Ricefish (Oryzias latipes 'Youkihi')

Meet the Youkihi medaka, a solid orange Japanese ricefish

The Youkihi medaka is an ornamental strain of the Japanese rice fish (Oryzias latipes) bred for one thing: an even, glowing orange from nose to tail. It is a slim fish with a flat back, a pearly belly, a large silvery eye and a dorsal fin set far back towards the tail, the shape of a fish that cruises just under the surface. If you want a bright, hardy and peaceful fish for a cool indoor aquarium that needs no heater in most homes, the Youkihi medaka is one of the best small fish you can choose.

Medaka belong to the ricefish family, Adrianichthyidae. Seriously Fish explains that the family now sits in the order Beloniformes alongside the halfbeaks and needlefish, although older books still file it among the killifish. That is why you will find medaka in our killifish and ricefish range and in the killifish category, and why its care is closer to that of a white cloud mountain minnow than to a tropical killifish. The genus name comes from the Greek for rice, after the paddy fields where the fish lives, and latipes is Latin for "wide-footed".

Check the names on the label

The strain name is Japanese. As an article in Japan Spotlight explains, Yōkihi is the Japanese name for Yang Guifei, the Tang-dynasty consort remembered as one of the four great beauties of ancient China. Medaka have been popular pets in Japan since the 17th century. Wikipedia's medaka article records 456 documented commercial strains and notes that genetic studies trace domesticated medaka mainly to one founder lineage from southern Japan.

The wild range depends on which authority you follow. Seriously Fish lists Japan, eastern Korea, Taiwan, eastern China, Laos and Vietnam, while noting that some mainland populations may belong to Oryzias sinensis. Wikipedia, following more recent splits, treats O. latipes as native to Japan alone. Either way, the fish in our tanks are aquarium-bred, not wild-caught.

Know the difference from glowing medaka

The Youkihi is a selectively bred colour strain. It is not the transgenic "glowing" medaka, which carries fluorescent proteins taken from a jellyfish or a coral. Seriously Fish notes that the glowing form is illegal to buy, sell or own in the European Union, and that the care of colour strains is essentially the same as for the wild fish.

Know exactly what you receive

One purchase supplies one live Youkihi medaka at the standard size we currently send, which is a young fish below adult size. Sex is not chosen or guaranteed. The depth of orange varies a little from fish to fish, and the photographs show the strain rather than the individual you will receive.

Buy enough for a confident group

• Group size: Seriously Fish says medaka look their best and are most confident in a group of eight or more.
• Mixing colours: other colour strains of the same species, such as our Tomoki Medaka Ricefish, school happily with Youkihi but will interbreed with them.
• Small tanks: a group of medaka is one of the brightest choices among our nano tank fish, provided the tank meets the base size below.

Read our delivery and Live Arrival Guarantee information before you order, and make sure the aquarium is filtered, matured and at a steady temperature before the fish arrive.

Give a 4 cm fish a 45 cm aquarium

Seriously Fish gives a maximum standard length (snout to the base of the tail) of 36 mm, FishBase gives 4.0 cm, and Wikipedia about 3.6 cm. Add the tail and an adult measures a little over 4 cm. It is a small fish, but an active one that lives in a group, so it needs swimming room rather than a bowl.

For a group, Seriously Fish recommends a base of at least 45 × 30 cm, about 41 litres. A longer tank, or one with a large surface area, gives a group more room to cruise near the top and more space for the planting that helps fry survive.

Run a quick space check

1. Measure the base: 45 × 30 cm or larger for a group of eight.
2. Count every fish: our stocking calculator shows how much room is left once tank mates are added.
3. Leave planting space: about a third of the tank for dense plants, the rest open water.
4. Fit a cover glass: it reduces evaporation and keeps the water temperature steadier in an unheated room.

Keep it cool: the unheated indoor aquarium

Medaka are temperate to subtropical fish. Seriously Fish gives 16–22°C and warns that keeping them permanently warm is likely to shorten their lives. FishBase gives 18–24°C. Wikipedia notes that the species survives anything from 0 to 42°C but prefers 15–28°C, a tolerance that explains its reputation for toughness but is not a range to aim for.

OATA's guide to unheated aquaria explains that most tropical fish need at least 22°C and a heater, while a number of smaller species do well in unheated indoor tanks, provided a filter is fitted. It advises keeping such tanks away from draughts, radiators, direct sunlight, loud noise and vibration so the temperature does not swing. Seriously Fish suggests a heater of 0–25 W for a 41 litre tank in a room around 20°C, which in practice means no heater in many heated homes.

Decide whether you need a heater

Our aquarium temperature guide explains how to read a thermometer and set a heater.

Match the water and keep it stable

Medaka are adaptable. Seriously Fish gives pH 6.5–8.5 and 5–25 dGH, and FishBase gives pH 7.0–8.0 and a hardness of 9–19 dH. The species is found from rice paddies and slow streams to tidal pools, and FishBase records that it tolerates a wide range of salinity. For an aquarium-bred strain such as the Youkihi, ordinary dechlorinated tap water is the starting point; Seriously Fish's advice about added salt concerns wild-collected fish, whose needs depend on where they were caught.

Match your tap water

Much of southern and eastern England has hard, alkaline tap water that suits medaka well once it is treated with a conditioner. Supplies in Scotland, Wales and parts of northern and south-west England are often softer, and medaka still cope inside Seriously Fish's range. OATA suggests checking your water company's online postcode checker. Compare the result with our water matcher and read our water chemistry guide for what each number means.

Keep medaka indoors and never release them

In Japan, medaka are often kept outdoors in bowls and tubs. In Great Britain the rules are different. GOV.UK guidance says a licence is needed to introduce and keep non-native fish, with exemptions for certain species kept in indoor aquaria and a short list of species for garden ponds. OATA states that the fish it recommends for unheated aquaria are not legally permitted outdoors for biosecurity reasons and must stay in indoor tanks. We sell the Youkihi medaka for indoor aquariums. If you are considering an outdoor container, check with the Fish Health Inspectorate first.

Never release medaka or any aquarium fish into the wild. Wikipedia reports that domestic orange medaka have already mixed with wild populations in Japan: in 2011, about 15% of wild-caught fish in Nara carried a genetic marker from the orange "himedaka" strain.

Get the patio-pot look indoors

• Choose a low, wide tank and view it from above as well as from the front; an orange fish stands out against dark gravel.
• Float a few broad leaves or floating plants to give shade, as in a summer tub.
• Use a gentle sponge or hang-on filter so the surface stays calm.

Plant densely and leave open water

Seriously Fish recommends a heavily planted aquarium with a dark substrate, dense patches of vegetation and open areas, with twisted roots, bogwood and plants at the surface. The dark floor matters for a colour strain: orange looks richer over dark gravel or sand than over a pale base. Fine-leaved plants double as spawning sites.

Zone the tank

Filter gently and keep a weekly routine

Seriously Fish suggests a filter rated at four to five times the tank volume per hour, about 165–205 litres per hour for a 41 litre aquarium. Medaka come from still and slow waters, so spread the outflow across the surface or use a sponge filter rather than creating a strong current. Our guide to aquarium filtration compares the options.

Follow a weekly routine

1. Test ammonia, nitrite, nitrate and pH; OATA advises testing at least once a week.
2. Change about 25% of the water with conditioned tap water at the same temperature.
3. Siphon waste from open areas of the substrate.
4. Rinse filter sponges in removed tank water, never under the tap, to protect the filter bacteria.
5. Trim plants so open swimming space stays clear.

Feed small, meaty meals every day

In the wild, Seriously Fish describes the medaka as a micropredator feeding on small insects, worms, crustaceans and other zooplankton, and Wikipedia notes that it eats young mosquito larvae. FishBase classes it as omnivorous. In the aquarium it accepts suitably sized dried foods, and Seriously Fish recommends small live or frozen foods every day alongside a good flake or granule.

Build one measured meal

• Staple: a fine flake or micro-granule that floats for a while, because medaka feed at and just under the surface.
• Daily extra: a small portion of Daphnia, brine shrimp (Artemia) or chopped bloodworm.
• Portion: only what the group eats within a couple of minutes, once or twice a day.
• Cool weeks: fish eat less in cooler water, so reduce portions if the tank drops towards 16°C.

Settle your ricefish in during the first week

Add medaka only to a filtered aquarium with ammonia and nitrite at zero. OATA describes the floating-bag method: switch off the aquarium light, float the sealed bag for about 10 minutes to match temperatures, add small amounts of tank water over up to 20 minutes, then net the fish out with as little bag water as possible. Our guide on how to acclimatise new fish shows each step.

Use first-week checkpoints

Choose calm tank mates that like it cool

Seriously Fish describes the medaka as generally peaceful but not an ideal fit for many community tanks because of its cool-water needs. It suggests small species with similar requirements, such as some halfbeaks (Dermogenys), blue-eyes (Pseudomugil) and bumblebee gobies, plus many neritid snails. It advises keeping medaka alone if you want to breed them, and never with other Oryzias species, because hybridisation has been shown in laboratory experiments.

Screen each companion

White clouds share the same cool conditions: OATA gives them 17–23°C. Read our White Cloud Mountain Minnow guide and our killifish guide for more cool-water ideas, and check any mix with our tank mates tool.

Breed Youkihi medaka at home

Seriously Fish calls medaka easy to breed and fairly prolific. Spawning usually happens early in the morning, when males darken and defend small temporary territories. The female releases a cluster of adhesive eggs that hangs from her body for a while before she brushes the eggs onto plants singly or in small clumps. Healthy females can spawn every few days, or daily. Wikipedia gives 10–20 eggs per spawning and notes that wild fish breed from spring through summer.

Tell males from females

Follow the egg-to-fry timeline

To keep the orange colour true, breed Youkihi only with Youkihi. Fry from mixed strains will show a variety of colours.

Watch these health signs

OATA describes a healthy fish as having clear, bright eyes, undamaged fins, intact scales and steady breathing. Signs of trouble include clamped fins, flicking against decor, shimmying, hanging at the surface, white spots or fluffy growths, and changes in feeding or colour. Wikipedia gives an average lifespan of about two years in the wild and three to five years in laboratories, and Seriously Fish's warning about permanent warmth is the main thing you control.

Pair each sign with an action

Answer Youkihi medaka buying questions

Is the Youkihi medaka a killifish?

Not quite. It is a ricefish, family Adrianichthyidae, which Seriously Fish places with the halfbeaks and needlefish in the order Beloniformes. Shops and older books often list medaka with the killifish, which is why you find it in that part of our range.

Does a Youkihi medaka need a heater?

Usually not. Seriously Fish gives 16–22°C, so a tank in a heated living room often needs no heater. Use a small thermostatic heater only if the room falls below about 16°C in winter, and avoid keeping medaka permanently warm, which Seriously Fish says is likely to shorten their lives.

How big does a Youkihi medaka get?

About 3.6–4 cm. Seriously Fish gives 36 mm standard length and FishBase 4.0 cm, both measured without the tail.

How many should I keep?

Eight or more. Seriously Fish says medaka look their best and are most confident in a group of that size, in a tank with a base of at least 45 × 30 cm.

Can I keep Youkihi medaka in a garden pond or patio tub?

Not without checking the rules. Medaka are non-native in Great Britain, and GOV.UK guidance requires a licence to keep many non-native fish unless the species and the way it is kept are exempt. OATA says the fish it lists for unheated aquaria must stay indoors for biosecurity reasons. We sell this fish for indoor aquariums.

Can it live with other medaka colours?

Yes. Other colour strains of Oryzias latipes, such as our Tomoki Medaka, are the same species and get on well. They will interbreed, so keep strains apart if you want pure orange offspring. Do not mix different Oryzias species, which Seriously Fish says can hybridise.

Is the Youkihi a genetically modified "glowing" fish?

No. It is a selectively bred colour strain. The transgenic glowing medaka is a different fish, and Seriously Fish notes that it is illegal to buy, sell or own in the European Union.

Will it breed in my aquarium?

Very likely. Healthy females can spawn every few days, or even daily. The adults ignore the eggs but eat free-swimming fry, so dense fine-leaved plants or a separate rearing container will save more young.

How many fish come with one purchase?

One live Youkihi medaka at the standard size we currently send, which is a young fish. Sex is not chosen or guaranteed.

Consult the care references

• Seriously Fish: Oryzias latipes — size, tank base, temperature, pH, hardness, filtration, planting, diet, tank mates, sexing, breeding and the golden and glowing forms.
• FishBase: Oryzias latipes — maximum standard length, temperature, pH and hardness, range, habitat, salinity tolerance and IUCN status.
• Wikipedia: Japanese rice fish — native range after recent species splits, temperature tolerance, egg numbers and hatching time, lifespan and the history of medaka keeping in Japan.
• Japan Spotlight (March/April 2022): The Legend of Yokihi — the meaning of the name Yōkihi (Yang Guifei) in Japan.
• OATA: Indoor coldwater fish, unheated aquarium inhabitants (September 2022) — which fish suit unheated indoor aquaria, where to site the tank and why such fish stay indoors.
• OATA: How to understand and test water quality in a freshwater aquarium — zero ammonia and nitrite, minimum oxygen, weekly testing and how to deal with tap-water chlorine.
• OATA: How to look after guppies and mollies — UK livebearer guidance on water quality, feeding, weekly water changes, acclimatisation and health signs.
• GOV.UK: Introduce or keep non-native fish and shellfish — UK government rules on keeping non-native fish, indoor aquaria and garden ponds.
• Tropical Fish Co delivery policy — current dispatch days and Live Arrival Guarantee terms.