
The guy who kept the arapaimas have them only in a small tank, with no room for turning or even moving. He got sued. =) The aquarium I'm interning in have 3 arapiamas in a huge tanks, with 2 garfishs, several pacus and several huge catfishs. But I,personally, still think that its a little overcrowded. If a government aided/funded aquarium have slight problem in rearing these monsters, let alone some guy in his own small aquarium.
bottom line: leave those giants out in nature, unless you're capable.