Can Least Killifish (Heterandria) and Northern Mountain Swordtail Live Together?

Least Killifish (Heterandria) 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 Least Killifish (Heterandria).

Least Killifish (Heterandria) and Northern Mountain Swordtail both frequent the Top (Surface) and Middle (Open Water) 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: Northern Mountain Swordtail 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 20°C and 25°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 8–20 dGH. Regular testing will ensure conditions stay within this range.

To house Least Killifish (Heterandria) 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.

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.

Least Killifish (Heterandria) prefers Low (Still Water) flow while Northern Mountain Swordtail needs High (River/Stream) flow. Consider positioning filter outlets and using baffles to create zones of different current intensity within the same tank.