An 8x3 aquarium will be sufficient long term for albino silver arowana, as they really only max out at 3 foot. Although in some rarer cases, they can grow past the 3 foot mark, so you should always be ready for that.
I have actually since decided not to put him in the 8x3 but in a nice big pool pond instead. It would be less aesthetically pleasing, but it will give my arowana more room to swim around. I have also became a strong believer that arowana should be in ponds and not in tanks.
