How can I calm these nervous fish?

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Bob Mehen advises a reader on keeping Silver sharks in an aquarium.

Q: I recently purchased two Silver sharks for my 125 l aquarium. They are about 6cm in size and eat well but seem very nervous. They dash around the tank a lot and it makes the rest of my fish scatter all over the place so I wonder if their behaviour is bothering them. I have plants and wood in the tank but wonder if they also might need a cave or two, or somewhere to hide. Please can you advise on how to settle these fish?

JOSH, VIA EMAIL

A: Bob says: The Silver shark, Balantiocheilos melanopterus, is one of those species which is somewhat iconic in the fishkeeping hobby and commonly sold, but is largely unsuitable for aquarium life. The skittish behaviour you describe is standard for the species and when they’re not inadvertently scaring their tank mates, they are usually crashing into the aquarium glass or leaping out of the tank at the slightest provocation.

They also grow in excess of 30cm and need to be kept in groups of five or more, so unfortunately your tank is far too small for them long term. Added to this, when these fish are kept in pairs, it’s common for the larger fish to bully the smaller one, often with fatal consequences. To keep Silver sharks, a tank with a footprint of 2m x 60cm would be the absolute minimum I’d recommend, which rules them out of most home aquaria.