Can Electric Blue Johannii and Least Killifish (Heterandria) Live Together?

Electric Blue Johannii is a semi-aggressive species (8/10), while Least Killifish (Heterandria) is peaceful (1/10). This notable difference means Electric Blue Johannii may occasionally assert dominance over Least Killifish (Heterandria).

Large aggression gap (7 points) between Electric Blue Johannii and Least Killifish (Heterandria) increases stress risk for the more peaceful fish.

Electric Blue Johannii and Least Killifish (Heterandria) both frequent the Middle (Open Water) and 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: Aggressive fish may bully and stress shy, slow-moving species. Hyperactive swimmers can stress shy, slow-moving species through constant motion.

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 Electric Blue Johannii and Least Killifish (Heterandria) 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 27°C. Aim for the midpoint of this range (around 25.5°C) for optimal comfort.

Both fish can tolerate a pH between 7.8 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 Electric Blue Johannii and Least Killifish (Heterandria) together, plan for an aquarium of at least 183 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: Sand (Sifters), Established Algae (Otocinclus), Plants - Densely covered, Plants - Floating. Meeting these requirements will help both species thrive.

Note a potential conflict in setup requirements: Electric Blue Johannii is a plant destroyer, which conflicts with Least Killifish (Heterandria)'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. Electric Blue Johannii 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.