An idea for a nice Arowanna tank with plants, is using a cork backing
on the back wall and covering it with either java/Xmas moss or
Bolbitus.
The dark cooler colors of these plants will greatly accent the scales
and colors of this fish.

You can even have a bare bottom tank or plants with low growing plants
etc. The big fish tend to tear up rooted plants and then trimming them
is tougher etc.
Nice logs, driftwood with some attached plants/moss etc works well
also on top of this idea. Java fern can be used for the bottom
exclusively attached to pieces of wood. No substrate needed.

CO2 can be diffused into the tank with a powerhead and spray bar along
the bottom back edge of the tank. The gas will be atomized as it hits
the impeller, then if you direct the gas up into the "Bolbitus wall",
it will act like a trickle maze for the CO2 and also directly supply
the plants all the way up. The plants wil also hide the powerhead, CO2
line and spray bar.

Regards,
Tom Barr