For advanced hobbyist care, why is the horizontal footprint of the aquarium more critical than its height for an adult arowana?

Because they primarily inhabit the top water column, needing ample horizontal space for movement and swimming

The care requirements for an adult arowana reflect its predatory nature and size. Since these fish are top-level feeders, their natural behavior involves inhabiting and moving within the surface layer of the water column. Consequently, the physical dimensions of their enclosure must prioritize surface area over vertical depth. An ample tank footprint—meaning long and wide dimensions—provides the necessary horizontal room for the fish to swim unimpeded, which is essential for its comfort and well-being. While they are famous jumpers requiring a tight hood, their daily activity budget is better served by having extensive horizontal territory rather than excessive vertical space, as they do not typically swim in deeper, lower regions of the tank.