an arowana would look awesome cruising around the top of an 8 foot tank with oscars in the mid section. clown loaches on the bottom would give you a really nice range through the whole height of the tank im my opinion. keep in mind that barramundi are predatory fish by nature, not aggresive. if they see something which they can fit in their mouth, or sometimes even if they cant fit it in their mouth, it will be considered prey. i think that watching a barramundi stalk its prey is one of the coolest things i have ever seen! good luck with the project