I had a black ghost knife grow from 2 inches to about 9 inches in 8 months. Ensure there is a gentle current so its constantly swimming and working out. Also check how much he can eat in about five minutes and divide that up into 4. Then feed him these smaller portions 4 times a day so that he can digest his food slower and more efficiently. Finally feed high quality food such as hikari sinking carnivore or massivore pellets, this stuff is fantastic for growth. If you dont think you could grow this fish to its full potential and have a aquarium large enough to maintain it then i wouldnt bother keeping it. I would recommend atleast a 5.5 foot by 1.5 foot by 1.5 foot. Trust me you need good filtration, messy fish