Can Harlequin Rasbora and Northern Mountain Swordtail Live Together?

Harlequin Rasbora is a peaceful species (1/10), while Northern Mountain Swordtail is generally mild-mannered (3/10). This modest difference means Northern Mountain Swordtail may occasionally assert dominance over Harlequin Rasbora.

Harlequin Rasbora and Northern Mountain Swordtail both frequent the Middle (Open Water) and 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.

Worth noting: Northern Mountain Swordtail is a known jumper, so a tight-fitting lid is essential.

The safe temperature window for both species falls between 22°C and 25°C. Aim for the midpoint of this range (around 23.5°C) for optimal comfort.

Both fish can tolerate a pH between 7 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 8–12 dGH. Regular testing will ensure conditions stay within this range.

To house Harlequin Rasbora and Northern Mountain Swordtail together, plan for an aquarium of at least 110 litres with a minimum length of 90 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, Leaf Litter/Blackwater. Meeting these requirements will help both species thrive.

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

Harlequin Rasbora and Northern Mountain Swordtail work best when the tank gives each fish enough room to use its preferred level of the aquarium and enough cover to avoid constant visual contact. The score matters, but the real-world success usually comes from matching their pace, keeping water stable, and resisting the urge to crowd the tank just because the pair looks safe on paper.