A nocturnal hunter, do not keep with smaller fish such as Neon Tetras. Black Ghost Knifefish are best kept one Knife to a tank. It is said that because of the electrical signals they give off to hunt, these can interfere with other Knife's signals in a confined space and lead to problems. They can also sometimes be territorial and pairs may often try to dominate one another. They are otherwise peaceful towards similar sized and larger fish, but should not be kept with aggressive fish or those that may fin-nip or bully the Knife.