Here we go again. I believe this question was posted about a month or so ago. I would recommend reviewing old posts to prevent reposting the same questions, but since you asked, here's my take on this question.
I personally wouldn't recommend anything smaller than a 240, and larger would definitely be better. That being said, there are others who have had success raising these fish in a 180.
The one thing to remember is that these guys are very powerful swimmers. When my two 10" monocs start fighting with each other, they make it all the way across my 8' long 200 in less than a second. So I definitely believe bigger is better.
