Gill, correct me if I'm wrong but I have always heard that a 55 is the minimum for a Bichir (excluding the larger species) with the width of a 75 making that size preferable.
I won't keep a 12"-13" fish in a tank that's only 12" wide. they may be able to live like that, but I don't consider that good fishkeeping. so while some might consider 18" width "preferable" I consider in necessary.