This topic has come up several times at my firm. Unless I missed it somewhere, The size of the joists was never stated, as well as the span of those joists. The joists would most likely carry the weight without adding support, but the floor will definelty have a lot more bounce to it. And like some else mentioned the floor will sag over time.
The 4x6 or 4x8 seems appropriate in this case, but the oldschool rule of thumb for sizing a beam based on the length of the opening does not take into accout some important factors(like the distance to another load bearing point). The steel beam will definetly work but is way overkill in my opinion, and not the most cost effective either. Doubling up the floor joists is another option but probably not the most ecomonical, the new beam and posts would require less material.
I would personly do the beam with the two posts. The placement of the beam is not that critical. I mean the tank does not need to be centered between the wall and the new beam as someone mentioned. Buy adding a new beam you not only help to support the added weight of the tank but you will also shorten the span of the original joist...thus making them stronger as well.
