The Silver Arowana will need an aquarium of 3ft. Width and 8ft. Length or larger for life. Anything not small enough to fit in the Aro's mouth like stated above or any over aggressive species.
Most may disagree with this but I feel 220 is adequate to grow out in...buy you a bit of time but they grow...and QUICKLY! My largest tank is 265...7ft long and I feel it's too small for my silver that's about 24in