I have found something I disagree with... it says black ghost knifefish and elephant noses are incompatible because they are both electrical fish... this is an old rumor that has been found to simply not be true.
Oddball kept a dozen elephant noses with a BGK for years, and I currently have them together in my 125 no problems.
However, elephant noses SHOULD be kept in groups of 6 or more ideally, to spread out the conspecific aggression, like tiger barbs.
One of the theories behind the compatibility is that ENs are from africa and give off a different type of electrical single than the BGK from SA. Also, it is thought that these fish can simply change their electrical signal if need be. No matter what, they ARE compatible together.