get a channa pleuro, they're usually pretty good with arowanas and i see them in monster community tanks all the time (well, the ones that aren't in the US anyways). Also pleuros get pretty nice electric blue colors so they look pretty good with arowanas. One more thing, they max out at 16" so a big silver will probably discourage an attack from the pleuro.
also, from what i hear, they are fairly docile for a snakehead.

also, from what i hear, they are fairly docile for a snakehead.