Can Cardinal Tetra and Mountain Rainbowfish Live Together?

Cardinal Tetra is a peaceful species (1/10), while Mountain Rainbowfish is peaceful (2/10). This modest difference means Mountain Rainbowfish may occasionally assert dominance over Cardinal Tetra.

Cardinal Tetra and Mountain Rainbowfish both frequent the 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: Mountain Rainbowfish is a known jumper, so a tight-fitting lid is essential.

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.

The pH requirements of Cardinal Tetra (4–7) and Mountain Rainbowfish (7.4–8.5) do not overlap, making it impossible to satisfy both species simultaneously.

For general hardness, the shared comfort zone is 8–8 dGH. Regular testing will ensure conditions stay within this range.

To house Cardinal Tetra and Mountain Rainbowfish 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: Leaf Litter/Blackwater, Driftwood (Digestion/Hiding), Plants - Densely covered. Meeting these requirements will help both species thrive.

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

Cardinal Tetra and Mountain Rainbowfish 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.