Which species os Arowana is it? Silvers and blacks are quite peaceful and will ignor fish that are too big to be eaten, jardinis (australian arowanas) can be aggressive, temperments vary from fish to fish. As long as the tankmates are at least 1/2 the size of the arowana and reasonably tall bodied they should be ok, fish with a slim profile will disapeer very quickly, other surface dwellers should be avoided too. To ensure your arowana stays peaceful avoid feeding large live foods, i feed my jardini on live crickets, hikari floating carnivor sticks, frozen cockles and bloodworms.