What can I keep with these knifefish?

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What fish can be kept with a Black ghost knifefish? Our fish expert discusses the best options…

I'd like to keep Black ghost knifefish. Are they sociable fish or do I need to limit it to just one in a 1.5m tank? Will a planted set-up be too bright for them, being nocturnal? Please could you give me some advice on feeding and also any fish I can keep with them that won't be eaten. Is there anything else I need to be aware of with these knifefish?

ADE JONES, VIA EMAIL

Neale replies: The Black ghost knifefish, Apteronotus albifrons, hails from South America. Naturally inhabiting deep river channels, it presumably prefers dark and gloomy environments where its electric sensory systems allows it to navigate and find food without problems.

In the aquarium these fish tend to be nocturnal, though whether this is their preference or simply a way to avoid bright light I cannot say, but settled specimens sometimes become more active during the day if there is plenty of shade and suitably peaceful tankmates. Plants themselves aren’t a problem, and if the tank has lots of floating vegetation, the knifefish will simply hang around in the shade during the daytime.

Wild fish feed primarily on insects, worms, and other small prey that they find hiding among the rocks and sunken wood. Under aquarium conditions they tend to ignore peaceful fish too large to be swallowed, but very small fish, such as Neons, would likely end up on the menu. Interestingly, wild fish probably form loose aggregations and use their electric organs to generate impulses that help fish communicate with one another. In the aquarium, their large adult size of 50–60cm makes it impossible to keep them in suitably large groups, and in twos or threes they invariably bully one another. While not too bothered about water chemistry, they are acutely sensitive to low oxygen levels, so under-stocking their aquarium is important.

If you must add tankmates, choose species that won’t compete for food. Herbivorous catfish, such as Panaque, would be obvious picks. You might also look at things like Anostomus that come from similar habitats, and while they may eat the same food, do so during the day, so shouldn’t cause any problems at night. Midwater schooling fish can be helpful, but select species that aren’t nippy or so small they might be eaten. Ex-Brochis catfish also work well, but they feed on similar foods, particularly when young, so it’s a good idea to get the knifefish feeding well before adding these.