I have seen them in LARGE systems together, I am talking three hundred gallons on the small side and as much as ten THOUSAND gallon tanks at public aquariums with amazon biotopes. They have tons of hiding spots, and they are well equipped to handle these fish. I know for a fact that arowanas can attain four and five foot lengths. Anything less is stunted growth. Go ahead and do it, I know many people with arowanas living miserable lives in 55 gallons (55, 90, 125 gallons are probably all the same to an arowana, or a tarpon, or an oar-fish for that matter) so I am not preaching here or trying to deter you, but maybe someone out there will learn something by reading this later on.