Can Harlequin Rasbora and Pseudotropheus sp. Kingsizei Live Together?

Harlequin Rasbora is a peaceful species (1/10), while Pseudotropheus sp. Kingsizei is semi-aggressive (8/10). This notable difference means Pseudotropheus sp. Kingsizei may occasionally assert dominance over Harlequin Rasbora.

Large aggression gap (7 points) between Harlequin Rasbora and Pseudotropheus sp. Kingsizei increases stress risk for the more peaceful fish.

Harlequin Rasbora and Pseudotropheus sp. Kingsizei 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: Provide a significantly larger tank (at least 50% more volume than the minimum) to allow both Harlequin Rasbora and Pseudotropheus sp. Kingsizei to establish separate territories. Add plenty of line-of-sight breaks using driftwood, rocks, and dense planting to reduce territorial confrontations. A larger aquarium with ample hiding spots can help diffuse aggression and give the less dominant fish space to retreat.

The safe temperature window for both species falls between 24°C and 28°C. Aim for the midpoint of this range (around 26.0°C) for optimal comfort.

Both fish can tolerate a pH between 7.5 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 10–12 dGH. Regular testing will ensure conditions stay within this range.

To house Harlequin Rasbora and Pseudotropheus sp. Kingsizei together, plan for an aquarium of at least 253 litres with a minimum length of 120 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, Leaf Litter/Blackwater, Sand (Sifters), Established Algae (Otocinclus). Meeting these requirements will help both species thrive.

Note a potential conflict in setup requirements: Pseudotropheus sp. Kingsizei is a plant destroyer, which conflicts with Harlequin Rasbora'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. Harlequin Rasbora and Pseudotropheus sp. Kingsizei 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.