Most of the time, they're even wrong for slow movers such as lionfish, and beyond mere volume, one often needs to consider tank footprint. For instance, a full grown P. volitans does MUCH better in a 100 gal tank that is 24" front-to-back than a std. 100 gal that is only 18" wide, as they require room to turn around and maneuver due to their pectoral spread. One should also consider how a given setup is aquascaped, as rockwork not only takes up "fish space", but reduces overall volume.
It's amazing how a fish's behavior changes when they have enuff room. For instance, lions go from sitting around a lot to actually swimming and just hanging in the water column.
