Assuming silver arowana, but info generally fits others too. With the silver aro you will need to provide a tank with a footprint that is 3x the aro in length and at least equal the aro in width/depth. The height of the tank, 24" minimum for a full grown aro. Silver aro grow rather quickly, on average 1-1.5" per month with growth spurts of up to 3" per month not being uncommon. The average silver will continue this high rate of growth up the 24-25" range, it growth from then on slows to just a couple inches per year. Better figure for at least 30" adult silver though more then likely will get larger with proper care.
Arowana are somewhat skittish, they need room to move when spooked. Many time when spooked they jump, number one cause of death. Tank top must be sealed and secured with weight or fasteners. When younger only a limited range of tankmates, once larger a wider variety can be attempted.
