Amen to giving the fish enough space. My tank (besides fish) is almost empty. I've got a rock pile in the middle of the tank and some driftwood pieces here and there, but big fish need lots of room to swim. Silver Arowanas are notorious for outgrowing tanks FAST and, sadly, too many grow up w/o enough room. People think these huge fish stay near the top of the water all the time and this is not true. Yes, a lot of Arowanas you see just float in one place at the top, but happy, healthy Arowanas are constant swimmers and too many suffer from bad muscle growth because an overcrowded or undersized tank does not allow them enough free-swim room. Award winning Koi keepers will tell you one of the most important things is to have a pond that's very deep. Giving big fish the ability to swim up and down, as well as back and forth, exersises different muscles and is VERY beneficial to overall health.
When my Arowana sees me, he stays at the top, waiting for food, but if I sneak into the room on him, he'll be all over the place (even skimming the gravel).
Just a tip: Most people put plants and other decor on either end of the tank to hide filter tubes or heaters, but I've found, especially with bigger fish, that putting obstacles in the middle, instead, gives them more room to swim side to side, and kind of acts as a natural divider, letting fish escape to the other side if necessary. My Arowana swims FAST when chasing feeders, and it's great to let him swim the full width of the tank.