there are two trains of thought with arowanas
in asia, very commonly, they will put the arowana by itself in a tank, with about 10 to 20% to spare of width in tank compared to its maximum body length, it just swims back and fourth.
2D really
OR giving the arowana chance to be able to jet around in different angles, so at leats 50% more compared to its width. so it can shoot around, even hunt or at least pretend to.
depends on you
i have seen massive high grade expensive arowanas in tanks just same as its width almost, and they are healthy, big and nice colours....
quality of life..thats another thing.
im sure a dog can survive, and be healthy being in a pen for its whole life but if you put a dog in a slightly bigger paddock or run, it will be happier right?
so yeah, depends really
i would say a 50cm aro in a 60 cm width.... sure thats fine, if you put it in a tank thats 80... would be much happier... just my opinion.