Can Freshwater Shark (Wallago) and Harlequin Rasbora Live Together?

Freshwater Shark (Wallago) is a highly aggressive species (10/10), while Harlequin Rasbora is peaceful (1/10). This notable difference means Freshwater Shark (Wallago) may occasionally assert dominance over Harlequin Rasbora.

Large aggression gap (9 points) between Freshwater Shark (Wallago) and Harlequin Rasbora increases stress risk for the more peaceful fish.

Freshwater Shark (Wallago) and Harlequin Rasbora 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: Freshwater Shark (Wallago) is a known jumper, so a tight-fitting lid is essential.

To improve the chances of a peaceful coexistence in this community tank: 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 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 2–12 dGH. Regular testing will ensure conditions stay within this range.

To house Freshwater Shark (Wallago) and Harlequin Rasbora together, plan for an aquarium of at least 5500 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: Sand (Sifters), Smooth Gravel (Sensitive Barbels), Plants - Densely covered, Leaf Litter/Blackwater. Meeting these requirements will help both species thrive.

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