Can Lorentz's Rainbowfish and Northern Mountain Swordtail Live Together?

Lorentz's Rainbowfish is a peaceful species (2/10), while Northern Mountain Swordtail is generally mild-mannered (3/10). This modest difference means Northern Mountain Swordtail may occasionally assert dominance over Lorentz's Rainbowfish.

Lorentz's Rainbowfish 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.

Worth noting: Lorentz's Rainbowfish is a known jumper, so a tight-fitting lid is essential.

There is no temperature overlap between Lorentz's Rainbowfish (26–30°C) and Northern Mountain Swordtail (20–25°C). This fundamental mismatch makes long-term cohabitation impractical.

Both fish can tolerate a pH between 7.2 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 8–15 dGH. Regular testing will ensure conditions stay within this range.

To house Lorentz's Rainbowfish and Northern Mountain Swordtail together, plan for an aquarium of at least 150 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, Plants - Floating, Driftwood (Digestion/Hiding), Leaf Litter/Blackwater. Meeting these requirements will help both species thrive.

Lorentz's Rainbowfish 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.

Lorentz's Rainbowfish and Northern Mountain Swordtail are the kind of pair that can look acceptable in a shop but become a maintenance problem at home. The main risk is not a single number in the chart; it is the way incompatible behavior, water needs, or pressure on space compounds over time once both fish are established in the same tank.