Can Colombian Tetra and Dusky Millions Fish Live Together?

Colombian Tetra is a generally mild-mannered species (4/10), while Dusky Millions Fish is peaceful (2/10). This modest difference means Colombian Tetra may occasionally assert dominance over Dusky Millions Fish.

Colombian Tetra and Dusky Millions Fish 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.

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 24°C and 24°C. Aim for the midpoint of this range (around 24.0°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 8–15 dGH. Regular testing will ensure conditions stay within this range.

To house Colombian Tetra and Dusky Millions Fish 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, Driftwood (Digestion/Hiding), Plants - Floating, Established Algae (Otocinclus). Meeting these requirements will help both species thrive.

Note a potential conflict in setup requirements: Dusky Millions Fish is a plant destroyer, which conflicts with Colombian Tetra's requirement for live plants. Compromise where possible or prioritise the more critical need.

Both species do well with moderate (standard) 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. Colombian Tetra and Dusky Millions Fish 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.