You can do full grown discus with black aro's as they're fairly peaceful fish. But they will out compete the discus for food and you'd need a large tank with a shoal of 10-12 adult sized discus, I'd say.
They will outgrow juvie discus and likely turn them into meals - so it's very very important you don't introduce the arowanna until the discus are adults (2-3 years of age).
I've seen some beautiful 300+ gallon tanks with discus, plants, a black arowana, and a huge shoal of rummynose and cardinals.
Rummynose are great for discus because the brightness of their noses is a good indication of water quality, when things start getting out of whack they will begin to dull out - and you can bet on the discus getting sick shortly thereafter.
Somthing else to watch out for is Hexamita, which discus are severely prone to. Unfortunately a lot of black aro's won't take anything but live foods for the first several weeks of being introduced into a new tank, which runs a large risk of introducing foreign bodies to the tank.
Check out
www.simplydiscus.com for some of the foremost expert's in the world. They're very friendly over there and have all kinds of advice and techniques you'd never even think of. Just takes a bit of reading.
