Mine, at about 8" is in a 180 and has been in there since I got it at 2", except for initial 20 gallon IP treatment for 3 weeks. It's a very active swimmer and always uses every inch of the tank. One thing many people don't seem to realize about Fahaka's is just how fast they can swim. While they may seem for the most part, to be sluggish fish, seeing them chase down AND CATCH a fleeing crayfish, is startling the first time and awe inspiring after that.
I had my first Fahaka in a 120 gallon, 60"x18"x26" tank and once it passed the 12" mark I felt badly about keeping in that small of a tank. I adopted it out to someone with a 180 and went Fahaka-less until I finally got a 180. While I respect many of the people that say a 120 at 48"x24"x24" or a 125 at 72"x18"x22" is a reasonable size tank for life, I personally feel that a 180 is much closer to a minimum size and if I could do it, I'd have mine in an even larger tank.