An arowana needs much more than a 3 1/2 foot 180 litre. If your talking about the sliver arowana then they can get to 3 feet long and you would need a tank in the region of 8ft long x 3ft wide x 2-3ft high. If it was a jardini then these get relatively smaller and so you could possibly get away with a tank around 6x2x2 as a bare minimum.