Can Japanese Ricefish (Medaka) and Least Killifish (Heterandria) Live Together?

Japanese Ricefish (Medaka) is a peaceful species (2/10), while Least Killifish (Heterandria) is peaceful (1/10). This modest difference means Japanese Ricefish (Medaka) may occasionally assert dominance over Least Killifish (Heterandria).

Japanese Ricefish (Medaka) and Least Killifish (Heterandria) 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.

Additional behavioural considerations to keep in mind: Hyperactive swimmers can stress shy, slow-moving species through constant motion.

Worth noting: Japanese Ricefish (Medaka) is a known jumper, so a tight-fitting lid is essential.

To improve the chances of a peaceful coexistence in this community tank: A larger aquarium with ample hiding spots can help diffuse aggression and give the less dominant fish space to retreat.

The safe temperature window for both species falls between 18°C and 27°C. Aim for the midpoint of this range (around 22.5°C) for optimal comfort.

Both fish can tolerate a pH between 7 and 8. Maintaining a stable value within this band is more important than hitting an exact number.

For general hardness, the shared comfort zone is 5–20 dGH. Regular testing will ensure conditions stay within this range.

To house Japanese Ricefish (Medaka) and Least Killifish (Heterandria) together, plan for an aquarium of at least 23 litres with a minimum length of 30 cm. This accounts for the larger species' space requirements with an additional 20 % buffer to reduce territorial tension.

Because this pairing requires extra vigilance, include plenty of hiding spots — caves, driftwood, and dense plant clusters — so the less dominant fish can retreat when needed.

Specific environmental needs for this combination include: Plants - Densely covered, Plants - Floating. Meeting these requirements will help both species thrive.

Both species do well with low (still water) water movement, so a standard filtration setup rated for the tank volume should suffice.