Well, anything can be the most expensive fish. I can put my tetra up for 2.2 billion dollars making it the most expensive fish, but it doesnt mean somebody is going to buy it. The better question would be what is the most expensive fish ever bought. L-250s would be the most expensive small fish. I would say asian arowana especially golden crossbacks would be the priciest big fish. Asian arowana arent really endangered, they are just endangered in the wild. The reason why they arent sold in the US is because i think the US wants the farms to release a certain amount from their farm into the wild for a certain amount sold. No farm is going to do this because itll be useless. They will release some, and somebody else would just grab it out of the river and sell it themselves.