I think the arowana is still the most expensive fish. But honestly, I think this is just a common fish in aquarium. Try the polka dot stingray, its looks awesome inside the aquarium.

Arowana I believe is a status symbol in some asian countries and can be very expensive. Here is a link to an interesting story that talks about how some arowana owners even get cosmetic surgery for their fish!

https://www.nytimes.com/2018/03/27/world/asia/sing...

"One fish was even rumored to have been bought by a Chinese Communist Party official for $300,000. So for most owners, the cost of an eyelift ($90) or a chin job ($60) for their pet fish is pocket change."

Crazy stuff... I you are not counting super large fish like sharks, rays, etc... then I would agree that the arowana is the most expensive... AT THE MOMENT!