Black Ghost Knife Fish (Apteronotus albifrons)Black Ghost Knife Fish are fascinating, nocturnal fish known for their sleek, jet black bodies and unique swimming style using an undulating fin along their underside. They are intelligent and can even recognize their owners over time. Minimum Tank Requirements: Tank Size: 300 litres or more Temperature: 24 28C pH: 6. 5 7. 5 Tankmates: Best kept with peaceful, medium to large fish like rainbowfish, larger tetras, gouramis, and non aggressive cichlids.