400 gallons is way off for an adult yes, but a 10,000+ is not absolutley necessary. If you could find a tank with wide dimensions, especially if it is VERY long and VERY wide, you could probably keep one in a good size 6,000 gallon tank or so.
Its kind of a misconception on people's part, they assume because they can be kept in an aquarium, they will not get big, but they do, really up to 7 feet. I have read about many getting them, but it is ridiculous what size tanks they keep them in!