My $0.02...
Arowanas grow REALLY big. They will quickly outgrow any tank smaller than 1.5 metres in length. They are natives of the Amazon floodplains. I used to work in an aquarium warehouse, we sold them as 10 cm juveniles for about $20 CDN. They are good jumpers, so you'll need a strong cover for the aquarium. Adults usually feed by jumping out of the water and catching insects, etc. on low-lying branches and leaves above the water. They are quite sensitive to water quality as well, so you'll need to keep the water clean and well-filtered.
I'd echo the thoughts on piranhas. They are quite boring most of the time, don't have the personalities the oscars do. I had an oscar in my home tank for over 15 years. When he died, I was surprised how sad I was...he had quite a personality. When I would feed him 10 goldfish, he'd eat 3-5 right away and then leisurely hunt the rest over about a week or so.
As for filtration, I'd recommend a Fluval for tanks over 100 gals, under that a good power filter is best. Undergravel filters don't work well for most tropical tanks.