My Managuense in my 125g is only about 10", and i don't expect it to get much larger.
These guys have BIG MOUTHS! they remind me of a bass.
Mine's very lazy though and just sits in its cave until food time, or when it needs to explore. It also lets the Jack Dempsey push it around a lot, even though its quite a bit bigger than the JD.
Pretty fish though. I like the purple sheen they have.