Japanese Rice Fish: Complete Care Guide (Tank Size + More)

Charlie Morton

Charlie Morton


Japanese rice fish

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The Japanese rice fish, or ‘medaka ricefish,’ has been attracting considerable attention over the last few years. Although these tiny fish have been available in the aquarium trade for decades, they’ve suddenly become one of the most trendy species in fish keeping.

Thankfully, the Japanese ricefish is up to the challenge of being kept by millions of first-time fish keepers! They’re incredibly hardy, breed quickly, and can even be kept in coldwater fish tanks, as well as heated aquaria.

Now, let’s learn about how to take the very best care of them!

Japanese Rice Fish at a Glance

Japanese Rice Fish Info
Scientific NamesOryzias latipes, Aplocheilus latipes, Poecilia latipes
Common NamesJapanese ricefish, Japanese Medaka, Japanese killifish, Youkihi Medaka Ricefish
Minimum Tank Size10 gallons
Level of CareEasy / Beginner Friendly
Ease of BreedingEasy
Size1.2 – 1.6 inches (3 – 4 cm)
Optimal Temperature64 – 82 °F (18 – 28 °?)
Optimal PH6.5 – 8.0
Optimal GH6 – 20
Life span1-4 years

Origin and Background

Firstly, the Japanese ricefish is not only from Japan! Oryzias latipes also inhabit parts of China, Taiwan, Korea, and Laos in Southeast Asia.

Also known as the ‘Medaka rice fish’, the wild species is remarkably adaptable and can be found in shallow, densely-vegetated ponds, swamps, slow-moving shallow rivers, ditches, brackish coastal waters, and irrigation canals. Their presence in rice paddies earned this fish its common name.

Despite their ability to adapt, the Medaka rice fish was listed as ‘threatened’ in 1999 by the Ministry of Environment in Japan owing to habitat loss.

Thankfully, fish in the aquarium trade are captive-bred, meaning that buying Japanese rice fish won’t affect their wild populations.

Size and Appearance

Japanese rice fish are among the smallest of all aquarium fish. At only 1.2 – 1.6 inches, they’re only slightly larger than an ember tetra.

Medaka rice fish are less colorful than some tropical fish, but many aquarists appreciate their modest beauty. Their body is mostly fairly translucent, and endless color variations have been developed by aquarium breeders.

Japanese rice fish have an unusual, elegantly-shaped body, with a long back and dorsal fin further to the rear than most fish.

Color Variations

You can now find Japanese rice fish in white, yellow, golden, red, pink, brown, olive, metallic, greenish, and blue color morphs. While some forms are described as ‘opaque,’ I’ve yet to see one that’s not at least slightly transparent!

Because they’re sometimes kept in ponds, some medaka have also been bred to be more attractive when viewed from above, like koi fish.

The recent worldwide craze for medaka has made breeding new color morphs big business, with select specimens sometimes selling for hundreds of dollars!

Japanese Rice Fish Care Guide

Tank Size

Japanese rice fish can be classed as social or schooling fish. Either way, you’ll need at least 6 for them to feel safe and happy.

Let’s use the one gallon per inch of fish rule to calculate the appropriate tank volume for a group of 6. Since their average size is around 1.5 inches, we multiply 1.5 inches x 6 and get 9.

Therefore, the smallest tank for a group of 6 medaka rice fish is 9 gallons. If you want to keep a larger group of them or other fish with them, however, you’ll need a larger tank.

It should also be stressed that larger tanks make it easier to maintain stable water parameters, so for first-time fish keepers, I’d recommend a 20- or 30-gallon tank.

Tank Environment

To create the ideal aquarium environment for the Japanese rice fish, we need to understand and replicate their natural environment.

In their natural habitats, Japanese rice fish live in fairly stagnant waters with dense tangles of aquatic plants. There would also often be rocks and pieces of driftwood for them to hide among.

They’ll be happiest then if we create a tank with a similar environment at home. Amazon sword, Java fern, Java moss, and Anacharis are all excellent beginner plants for aquariums and will go a long way to making the tank more beautiful.

Aquarium-safe rocks can be used to create caves, and pieces of driftwood can look extremely decorative when surrounded by live plants.

Water Temperature

Japanese rice fish aren’t your typical tropical fish! While they can withstand high temperatures of up to 82°F, they’re also happy in cooler water, down to 64°F. This means they can be kept in unheated, coldwater fish tanks, as well as tropical tanks with an aquarium heater.

The Japanese rice fish’s outdoor presence in Hokkaido, the snowy island in the north of Japan, shows how incredibly hardy these fish are! However, temperatures lower than the mid-60s Fahrenheit might induce hibernation.

Interestingly, Japanese rice fish have been shown to live much longer in cooler water temperatures but breed more prolifically at higher temperatures. When kept in water above 78°F, they’ll often exhaust themselves from constant breeding and die within a year.

Like other fish, Japanese Rice Fish are sensitive to sudden changes in water temperature. Acclimatizing them properly in a new tank is essential to avoid thermal shock. Partial water changes should also be made with treated water of matching temperature.

Water Parameters

Japanese Rice Fish

Renowned for tolerating a wide range of water parameters. Japanese rice fish are said to tolerate a pH of anywhere between 6.0  to 9.0. They’ll be more comfortable in water that’s closer to neutral, however, with a pH of between 6.5 – 8.

Medaka is equally unfussy about water hardness, but a dGH of between 6-20 is optimal.

Amazingly, the Japanese rice fish is so adaptable that it can even tolerate full-strength seawater! Therefore, it can be kept in tanks with brackish water fish without problems.


As with keeping any fish, an efficient aquarium filter is essential for keeping Japanese rice fish.

Sponge filters are suitable for tanks up to 10 gallons, but for larger tanks, invest in a reliable hang-on-back filter (HOB) or internal power filter.

Because they come from stagnant rice fields and slow-moving waters, Japanese rice fish can get stressed by a strong filter flow.

Some filters allow you to manually adjust the flow rate. On other models, you can reduce the water current by using an aquarium spray bar or lily pipe.

Diet and Feeding

Japanese rice fish are omnivorous and generally unfussy in their eating habits. In the wild, they feed largely from the surface on different types of plankton and small invertebrates.

In the aquarium, they’ll accept almost any type of fish food, provided they can fit it in their tiny mouths! This means that, while soft dried fish foods like fish flakes are ideal for them, fish pellets might prove too big and hard for them to swallow and could lead to choking.

As with any fish, dried fish foods should always be supplemented with fresh and frozen foods, such as bloodworms, tubifex worms, daphnia, brine shrimp, and mosquito larvae.

If there are larger, surface-feeding fish in the tank, just be sure that your Japanese rice fish are getting enough food. Fast-moving surface feeders like zebra danios could easily snap up most of the food before your rice fish have had a chance!

On the other hand, it’s important to avoid overfeeding. It’s generally recommended to feed your fish twice a day, with no more than they can eat in 2 minutes.

Overfeeding is one of the most common causes of water quality and health problems in aquarium fish. Even if your fish seem hungry, be careful not to feed too much!


In general, Medaka ricefish are a peaceful fish species. While many people prefer to keep them in a species-only tank, they can also be kept in community tanks with other peaceful species.

They will, however, sometimes fight for food and dominance within their own school. As with many fish, males are slightly more aggressive and dominant than females. Although they may occasionally nip the fins of each other, ricefish will rarely nip at other fish.

Japanese rice fish are fairly timid fish that can get startled easily. When scared, they may even try to leap out of the tank!

To ensure they’re calm and feeling safe, keep them in groups of at least 6 with plenty of plants, and avoid keeping large or aggressive fish with them.

Compatible Tank Mates

Ricefish are among the smallest and most delicate of all aquarium fish, leading many aquarium owners to keep them without other fish species. If you do keep them in a community aquarium, they need to be kept with other small, docile fish species only.

Small rasboras like Harlequin and chili rasboras make a good choice for other schooling fish. Small livebearers like guppies, endlers, and platies are another possibility, as are clown killifish.

Small tetras like neon and rummy nose tetras make acceptable tank mates as long as they feel safe and don’t start nipping at your rice fish’s fins. Keeping tetras in large groups helps to reduce aggression, and at less than an inch in size, the ember tetra is probably your safest bet.

As for bottom feeders, the Corydoras catfish family (cories) make an excellent choice and will also help keep the tank clean. Other clean-up crew tank mates like nerite snails, cherry shrimp, and Amano shrimp make ideal tank mates.

Never keep Japanese rice fish with larger fish or aggressive fish like cichlids, barbs, goldfish, gouramis, paradise fish, or even betta fish. Your medaka will likely get eaten!

Health and Disease

Medaka ricefish are remarkably robust and adaptable and will tolerate many beginner’s mistakes before getting sick.

Every fish has its limits, though, and Medaka are still susceptible to the same diseases and health conditions of other fish when they become stressed or when tank conditions are particularly poor.

Ich, velvet, and flukes are all common parasitic diseases that can lie dormant in an aquarium for years until a weakened fish or very poor water quality offers them a chance to strike.

Similarly, bacterial and fungal infections like columnaris and water molds only tend to affect fish that are already in a vulnerable state.

As mentioned earlier, Japanese Rice Fish have been known for fin nipping between each other when kept in small groups or when stressed. Nipped fins can lead to fin rot – a bacterial infection that can later invade the rest of the body.

Keeping Medaka in groups of 8 or more, with plenty of hiding places in clean water, is, therefore, the best defense against stress and unnecessary diseases.

Sexual Dimorphism

Japanese rice fish attain sexual maturity 2-3 months after hatching, when their body reaches around 1 inch in length. The visual differences between the sexes are subtle.

Males can be differentiated from female rice fish by their slimmer shape and longer dorsal and anal fins. Females have a more elongated pelvic fin and tend to be plumper, especially before spawning.

In male rice fish, the genital papilla is a short, slightly conical tube. Females have a bilobed (two-lobed) structure instead.


Japanese rice fish are egg scatterers and have a reputation for being easy to breed. They are also extremely unusual in the way that they can spawn every single day at dawn when conditions are right!

How To Breed Japanese Rice Fish

  • Keep male and female ricefish in a tank with Java moss or a spawning mop, which can be easily transferred to another tank.
  • Medaka can breed at any temperature above 59°F, but to initiate maximum spawning, set your thermostat to 77-82°F, and keep the tank lights on for at least 13 hours per day (lower temperatures reduce breeding rate. Reducing tank lighting to 8 hours per day will stop breeding).
  • Watch your ricefish at dawn for their courtship dance. Males chase females, displaying erect fins and bright colors. If the females accept the advances, the two will swim to the bottom of the tank and quiver side-by-side as they mate.
  • The sticky, fertilized 1mm eggs will hang onto the female’s genitals for a few hours before she deposits them among the live plants or spawning mop.
  • Eggs take between 7 – 14 days to hatch, depending on the water temperature. Parents won’t usually eat the eggs, but they will eat the fry once they’ve hatched, therefore move the spawning mop or Java moss to a designated breeding tank with a sponge filter to raise the fry.
  • Once they’re free swimming, the baby fish can be fed on baby brine shrimp, microworms, ground-up flake foods, and liquid fry foods. Keep larger and smaller batches apart, since larger fry can eat smaller ones!


Japanese rice fish

Medaka are short-lived fish whose lifespan seems to depend largely on water temperature.

Whereas fish living in the upper limits of their temperature range (around 80°F) usually only live for around 1 year, those living in unheated aquariums may frequently reach 4 years.

When kept in ponds, Japanese rice fish have been reported to live for a maximum of almost 5 years.

For them to reach their maximum lifespan at any temperature, it should go without saying that a high-quality diet, water, and care are essential.

Tank Maintenance

Here are some top tips for keeping your Japanese rice fish in optimal condition:

Japanese Rice Fish Buying Guide

Japanese rice fish are very unusual in that you can readily purchase them in the form of eggs rather than young fish.

If you do decide to buy eggs, raise them in their own tank using the pointers given at the end of the breeding section above.

The recent craze for Japanese rice fish has led to inflated prices and a race to breed new forms with exotic colors.

For an ordinary medaka, you could pay as little as $2 per fish, but for selected breeds, you could pay hundreds of dollars for a pair!

Final Thoughts

Medaka rice fish are some of the easiest fish for novice aquarists and can even be bred by beginners!

Their robust nature, small size, and elegant form make them an ideal candidate for those looking to get started in the aquarium hobby, as well as for advanced fish keepers looking for an easy fish to go to an aquascaped aquarium.

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