I am good friends with a lady who owns a marine store here called Seaquariums. She has shown me photos of tanks she custom builds for people.
One of the tanks she built is 5000G. The guy who owns it keeps sharks in it. I don't know what species of shark they are, but they only grow to 6 ft. This guy set up the tank and literally built his house around. With a tank that big, you can't just cut off your roof and tear out a room to make space for it. Well, I guess you could, but that would be too much work.
This tank is built into the wall of a room that I believe he has billiard tables set up in.
I would LOVE to have a 5000G tank, but I wouldn't keep sharks in it.
I'd be going for the red-tailed cats!