What is the average maximum length for a Black Ghost Knife fish? I've had one for about a year now, and its around 10 inches, and was wondering how much bigger it will get.
hmm.. yes.. they have the potential of reaching 20 inches.. I have never seen one that big.. (Red devil has a pretty big one?) I'd say you have a massive bgk if you get it hitting 15".. and that will take years..
They tend to average more along the lines of 16" in my opinion. A 75g would be the minimum I would suggest. You could probably keep them in 50g for a long while.
39 gallons is too small for the size he is at now... you could stunt him. If you upgrade him to a bigger tank now, he should grow faster and be happier and healthier. Remember you need a tank with some height to it, like a 150 gallon or bigger (220 would be sweet).