Having kept "REX", a silver arowana, from yolk sac till he outgrew my tank at a little better than 18" (he is now in a 300 at my local fish store) I would recommend against placing anything hard-edged or sharp, i.e., driftwood, sticks, rocks, decorations, etc. Aro's are violent when they feed and will injure themselves on such objects. I would recommend smooth rock, gravel and plants at the lower levels, but keep the mid and upper regions clear unless you want to use some floating plants at the surface. Also make sure the top is well covered and weighted. You would be surprised at how forcefully they jump and how small an opening they can get thru. Finding your prize fish flopping on the floor isn't a sight you will enjoy. Good luck and enjoy your monster fish.