Can Black Pacu and MacCulloch's Rainbowfish (Dwarf Rainbowfish) Live Together?

Black Pacu is a generally mild-mannered species (4/10), while MacCulloch's Rainbowfish (Dwarf Rainbowfish) is peaceful (2/10). This modest difference means Black Pacu may occasionally assert dominance over MacCulloch's Rainbowfish (Dwarf Rainbowfish).

Black Pacu and MacCulloch's Rainbowfish (Dwarf 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.

Worth noting: Black Pacu is a known jumper, so a tight-fitting lid is essential.

The safe temperature window for both species falls between 22°C and 28°C. Aim for the midpoint of this range (around 25.0°C) for optimal comfort.

Both fish can tolerate a pH between 6 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 5–15 dGH. Regular testing will ensure conditions stay within this range.

To house Black Pacu and MacCulloch's Rainbowfish (Dwarf Rainbowfish) together, plan for an aquarium of at least 3800 litres with a minimum length of 300 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: Smooth Gravel (Sensitive Barbels), Driftwood (Digestion/Hiding), Plants - Densely covered. Meeting these requirements will help both species thrive.

Note a potential conflict in setup requirements: Black Pacu is a plant destroyer, which conflicts with MacCulloch's Rainbowfish (Dwarf Rainbowfish)'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. Black Pacu and MacCulloch's Rainbowfish (Dwarf Rainbowfish) 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.