How long does it take for the black ghost knife fish to grow??? He's been in my tank now for a few months and the pleco he was bought before has passed him in growth .e and the pleco were purchased around 3 inches each. Now the pleco is around 6 inches and the BGK is only 3.5 inches! H They are in a 75gal tank with some angels and guppies that are thriving! I've feed him blood worms every day and have supplemented with brine shrimp & flake. I know that he's been getting food because i hand feed him. how long do they live? I can be patient , but it just seems that he should be further along in his growth!