[Myka]

Is the ceiling dropped in the basement or open floor joists or?

You could probably do it with a steel beam, but I would be inclined to use 2 or 3 smaller ones than one big one just to get a lower profile on the underside of the floor. You would have to use good hangers to mount the beam underneath and span at least the 2 joists under the tank and then at least another 2 on either side so the beam(s) would meet with 6 joists.