Full marine
I wish it was fresh though
Would give me time to grow it bigger before it went in to the main saltwater tank with the eel that would love to eat him
So he'll grow out for a couple months before he's in with the other fish
They require full marine conditions, so unless you want to take on this huge fish and learn the vast (and expensive) side of saltwater... I'd pass on this "opputunity".
I had one of thes guy's and he grew to 16 inch in one yr. I got him at 3 ". He is now a permanent fixture at out lfs because no one has a tank big enough or filtered enough for him. I love him dearly and visit him all the time. But in 1 yr he got to big for my 180gal tank. I did a lot of research while i had him and i beg you not to put him in fresh. It is not healthy for him and unfair to pur a full marine fish in a fresh envirnmant just because he "can" servive. It don't mean he will thrive. And be prepared. Mine in one yr was large enough to eat my full grown zebra eel. So watch out for your eel or other tank mates. He has also since killed 2 panther groupers at the pet store. Good luck.
people buy arapaima gigas, and that's not suitable for home aquarium, and neither are redtail catfish, pangasius sanitwongsi, alligator gars, etc... but people still keep them.
and on the thought that they "must" live in saltwater as adults, where does it say that its a must. You guys have a lot that you haven't seen, but just because you haven't seen it, doesn't mean that it doesn't exist or its impossible.