Sorry to hear, bro. Welcome to the club. I buried a dozen of these beauties over the last 10 years. A very hard fish to house. Still have just one last one left (18" in 240 gal) but it has started doing poorly in the last year. It's been trying to swim away through the glass for months now, rubbing a huge sore on the snout. Still feeds and right after goes back to trying to swim through the glass.
https://www.monsterfishkeepers.com/...ousseauxii-aka-dorado-catfish-4.678571/page-6
I feel these fish need a very large tank, like a few thousand gallons for my 18"-er, better with only one transparent wall / window... and absolutely the meekest and gentlest of tank mates if any at all... which is why I can't rehome mine into any other of my larger tanks, where it'd be nipped or bitten and kill itself or outright get killed by tank mates.
If it wasn't too small, yours could have done ok in your arowana haven pond but it too be a huge risk.
They too are so high strung and nervous that any rehoming can kill them (they will run into a wall and kill themselves). Couple this with an aggressive and cold welcome from the tank mates and chances are stacked heavy against the dourada.
These are the reasons we NEVER see large ones, over 1 foot, raised by MFKers or others. No one wants to dedicate one fish, even as stunning as this one, a swimming pool... or knows
the space and tank mate stringent, really life and death critical requirements.
So, from my firsthand experience this is more expected than not but also I know how this hurts.