Article on the Tiger Knife states this about tankmates and housing..
Aquarium: These electrical fishes are adapted for life in very dark peaty waters which don't even get much light during the day, so make sure the tank is very shady. Furnish the tank with bogwood and use a silver sand substrate. Males of the larger gymnotids can sometimes be aggressive, so be careful about mixing this with similar fish. Robust but placid cichlids, such as Heros, Acarichthys or Geophagus, should be safe tankmates. Given the large adult size, a tank of at least 120cm/4' is required for this fish, preferably much larger