Can Fiddler Crab (Brackish) and Least Killifish (Heterandria) Live Together?

Fiddler Crab (Brackish) is a generally mild-mannered species (3/10), while Least Killifish (Heterandria) is peaceful (1/10). This modest difference means Fiddler Crab (Brackish) may occasionally assert dominance over Least Killifish (Heterandria).

Fiddler Crab (Brackish) and Least Killifish (Heterandria) both frequent the Bottom (Substrate) 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.

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 24°C and 27°C. Aim for the midpoint of this range (around 25.5°C) for optimal comfort.

Both fish can tolerate a pH between 7.5 and 8. Maintaining a stable value within this band is more important than hitting an exact number.

For general hardness, the shared comfort zone is 10–20 dGH. Regular testing will ensure conditions stay within this range.

To house Fiddler Crab (Brackish) and Least Killifish (Heterandria) together, plan for an aquarium of at least 38 litres with a minimum length of 45 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), Driftwood (Digestion/Hiding), Plants - Densely covered, Plants - Floating. Meeting these requirements will help both species thrive.

Both species do well with low (still water) 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. Fiddler Crab (Brackish) and Least Killifish (Heterandria) 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.