Can Arapaima and Japanese Ricefish (Medaka) Live Together?
Arapaima and Japanese Ricefish (Medaka) are not recommended as tank mates due to piscivorous fish will prey on nano-sized tank mates.. Housing these species together is likely to result in stress, health problems, or direct harm to one or both fish.
At a Glance
Can your tank handle Arapaima and Japanese Ricefish (Medaka)?
Compatibility is only part of the answer. Test both fish with your real tank size, current stock, and maintenance needs to see if you have enough space and a safe stocking level before you add them.
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Behaviour & Temperament
Arapaima is a semi-aggressive species (7/10), while Japanese Ricefish (Medaka) is peaceful (2/10). This notable difference means Arapaima may occasionally assert dominance over Japanese Ricefish (Medaka).
Arapaima and Japanese Ricefish (Medaka) both frequent the Top (Surface) region of the setup. This overlap can increase competition for space, so providing ample room and cover is important to keep both species comfortable.
There is a critical concern: the Piscivore (Eats small/nano fish) behaviour of one fish directly conflicts with the Nano / Bite-sized (Predation Risk) trait of the other — piscivorous fish will prey on nano-sized tank mates..
Worth noting: Arapaima is a known jumper, so a tight-fitting lid is essential.
Water Parameters
The safe temperature window for both species falls between 24°C and 28°C. Aim for the midpoint of this range (around 26.0°C) for optimal comfort.
Both fish can tolerate a pH between 6.5 and 7.5. Maintaining a stable value within this band is more important than hitting an exact number.
For general hardness, the shared comfort zone is 5–15 dGH. Regular testing will ensure conditions stay within this range.
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Tank Setup
To house Arapaima and Japanese Ricefish (Medaka) together, plan for an aquarium of at least 15000 litres with a minimum length of 600 cm. This recommendation combines the stricter species minimums with a community-load allowance so the pairing has realistic long-term space and filtration headroom.
Providing a mix of open swimming space and sheltered areas with rocks, driftwood, or plants will keep both species comfortable and allow natural behaviour.
Specific environmental needs for this combination include: Plants - Densely covered, Plants - Floating. Meeting these requirements will help both species thrive.
Note a potential conflict in setup requirements: Arapaima is a plant destroyer, which conflicts with Japanese Ricefish (Medaka)'s requirement for live plants. Compromise where possible or prioritise the more critical need.
Both species do well with low (still water) water movement, so a standard filtration setup rated for the tank volume should suffice.
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Frequently Asked Questions
Can Arapaima and Japanese Ricefish (Medaka) live together?
No. This combination is not recommended. Key incompatibilities — such as critical behavioural conflicts — make cohabitation unsafe.
What size tank do Arapaima and Japanese Ricefish (Medaka) need?
A minimum of 15000 litres (tank length at least 600 cm) is recommended. This provides enough space for both species to establish their own areas and reduces the likelihood of territorial disputes.
What water temperature is best for Arapaima and Japanese Ricefish (Medaka) together?
Keep the aquarium between 24°C and 28°C. A target of around 26.0°C sits comfortably within both species' preferred ranges.
Are Arapaima or Japanese Ricefish (Medaka) aggressive?
Arapaima is semi-aggressive (7/10) and Japanese Ricefish (Medaka) is peaceful (2/10). The gap in aggression levels means the calmer fish may experience stress from the more assertive one.
What pH do Arapaima and Japanese Ricefish (Medaka) need?
Both species overlap in the 6.5–7.5 pH range. Consistency is key — avoid sudden swings, and test regularly to stay within this window.
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