Can Northern Mountain Swordtail and Red Eye Tetra Live Together?

Both Northern Mountain Swordtail and Red Eye Tetra are generally mild-mannered species with an aggression score of 3/10. Their matching temperaments mean neither fish is likely to dominate the other in a shared aquarium.

Northern Mountain Swordtail and Red Eye Tetra 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: 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: Keep fin nippers in larger groups to distribute nipping behaviour among conspecifics rather than tank mates.

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 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 Northern Mountain Swordtail and Red Eye Tetra 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. Meeting these requirements will help both species thrive.

Note a potential conflict in setup requirements: Red Eye Tetra is a plant destroyer, which conflicts with Northern Mountain Swordtail's requirement for live plants. Compromise where possible or prioritise the more critical need.

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

This pairing usually fails when the tank is treated like a standard mixed community and the caution flags are ignored. Northern Mountain Swordtail and Red Eye Tetra 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.