Rice Fish (Medaka)
Oryzias latipes
Min Tank Size
20L
Adult Size
4 cm
Lifespan
3 years
School Size
6+
About
Medaka come from Japan, Korea, and parts of China, where they've lived in rice paddies, slow streams, and roadside ditches for centuries. They're genuinely tiny, rarely hitting 4 cm, and they spend most of their time near the water's surface, darting around in loose groups.
What makes them stand out from most nano fish is their remarkable temperature tolerance. They'll handle anything from cool unheated tanks in the low teens to warm tropical setups pushing 28C, which makes them unusually flexible for aquarists working with unheated or outdoor containers. Water parameters don't need to be dialed in obsessively. Neutral to slightly alkaline pH suits them well, and they're reasonably forgiving about hardness as long as extremes are avoided. A gentle filter or a sponge filter is ideal since they're not strong swimmers and don't appreciate heavy flow.
Feeding is easy because they'll eat almost anything small enough to fit in their mouths: micro pellets, crushed flakes, baby brine shrimp, daphnia, and even biofilm they graze off plant surfaces. Japan has a passionate medaka hobby that has produced dozens of selectively bred color and fin varieties, from whites and golds to striking blacks and even longfin strains. Most of what you'll find outside Japan are the standard wild-type or basic gold forms, though specialty breeders are changing that.
The breeding behavior is one of the most distinctive things about this species. Females carry fertilized egg clusters attached to their belly for a few hours before depositing them on fine plants or spawning mops, giving you a short window to spot and collect them before other fish do.
Browse real medaka tank builds to see how hobbyists style outdoor tubs and minimalist nano setups around this species.
Water Parameters
Temperature
°CpH
GH
dGHKH
dKHSwimming Level
Flow Preference
Keeping multiple Rice Fish (Medaka) together
Rice Fish (Medaka) are shoaling fish and need company of their own kind. Keep a group of at least 6. Smaller groups leave them stressed, washed-out in color, and prone to hiding.
Compatibility
Medaka work well with other small peaceful species that share their temperature preferences. White cloud mountain minnows are a classic pairing, particularly in unheated or outdoor setups. Small livebearers, pygmy corydoras, and ember tetras in warmer tanks are all solid options. Avoid anything large or nippy since medaka are small and surface-oriented, which makes them easy targets for mid-sized aggressive fish. Cherry shrimp are generally fine tankmates, but very small juvenile shrimp may occasionally get eaten, so dense planting helps. In dedicated outdoor tubs, medaka are often kept alone or with small snail species, which works beautifully.
Commonly kept with
Species this one is most often paired withCommonly tried but avoid
Often paired, but shouldn't beCare Notes
The most common mistake is underfeeding. Their small mouths mean standard-sized flake food needs to be crushed finely, or they'll struggle to eat enough. Overfiltering is another issue since strong flow stresses them out and makes it harder for them to spawn. They breed readily, so without a plan for managing fry, population can grow faster than expected. Plants like hornwort or java moss double as both egg-catching surfaces and fry refuges. Outdoor keeping is rewarding but bring them inside before temperatures consistently drop below 8-10C.
Behavior & Aggression
Medaka are about as non-aggressive as freshwater fish get. Males will occasionally chase each other during spawning activity, but it rarely escalates to actual damage and usually resolves itself quickly once a pecking order is established. They don't nip fins, don't harass invertebrates in any persistent way, and largely ignore other species. The only real concern is that adults, including females, will eat their own eggs and newly hatched fry if given the chance. This isn't aggression in the traditional sense, just opportunistic feeding, but it's worth accounting for in a breeding setup.
Things to Know
- Prolific egg layer, eggs hatch in 10-14 days at warmer temps
- Adults may eat fry, use floating plants or a separate rearing container
- Many ornamental color varieties exist, but all behave identically
- Can be kept outdoors in warm months, but avoid extreme temperature swings
- A tight fitting lid is essential, they are known jumpers.
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