Can Arapaima and Reticulated Hillstream Loach Live Together?

Arapaima is a semi-aggressive species (7/10), while Reticulated Hillstream Loach is generally mild-mannered (3/10). This notable difference means Arapaima may occasionally assert dominance over Reticulated Hillstream Loach.

In terms of spatial distribution, Arapaima prefers the Top (Surface) and Middle (Open Water) zones, whereas Reticulated Hillstream Loach occupies the Bottom (Substrate) zone. Because they use different levels of the water column, territorial disputes are less likely and the tank will appear more balanced.

Worth noting: Arapaima is a known jumper, so a tight-fitting lid is essential.

To improve the chances of a peaceful coexistence in this community tank: Provide a significantly larger tank (at least 50% more volume than the minimum) to allow both Arapaima and Reticulated Hillstream Loach to establish separate territories. Add plenty of line-of-sight breaks using driftwood, rocks, and dense planting to reduce territorial confrontations. 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 24°C and 24°C. Aim for the midpoint of this range (around 24.0°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 4–15 dGH. Regular testing will ensure conditions stay within this range.

To house Arapaima and Reticulated Hillstream Loach 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.

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: Smooth Gravel (Sensitive Barbels), Established Algae (Otocinclus), Plants - lighly covered. Meeting these requirements will help both species thrive.

Note a potential conflict in setup requirements: Arapaima is a plant destroyer, which conflicts with Reticulated Hillstream Loach's requirement for live plants. Compromise where possible or prioritise the more critical need.

Arapaima prefers Low (Still Water) flow while Reticulated Hillstream Loach needs High (River/Stream) flow. Consider positioning filter outlets and using baffles to create zones of different current intensity within the same tank.

This pairing usually fails when the tank is treated like a standard mixed community and the caution flags are ignored. Arapaima and Reticulated Hillstream Loach need extra room, more cover, and closer observation during the first few weeks so small aggression, feeding, or territory issues do not snowball into a long-term problem.