Not very many people have or would build a tank large enough for those monsters. I saw a few pics on another site a while back, some guy in CA
had built a really huge floor-to-ceiling setup into his house, L-shaped, that holds several thousand gallons of water. Cost was way too much for the average fish keeper. He had a small boat to maintain the tank, said the fish, including an arapaima, had bitten holes in the inflatable rubber boat he started with.
A friend who owns an lfs
had one for a while, until someone forgot to cover the tank after feeding. It jumped out during the night.
They would be a safe fish to keep with a few tiger shovelnose, at least the catfish wouldn't have arapaima for dinner.
The link to the 50,000 gal
tank is posted on the forum, if you want to take a look.