If I were you, I would basically build a pier system underneath the loaded area of the floor. I would also connect the floor joists to the wall studs laterally.
Basically a laced post box system composed of 4 4x4s laced together has a safety factor of 32,000lbs. This would be more than adequate. This resists all 3 dimensions of load, as well as racking & twisting.
Like this..... just prettier http://www.flickr.com/photos/teex/4116014818/lightbox/
Or if you're feeling cheap, just use some diagonal bracing into the wall studs for added support.
EDIT: I see you changed your mind. Good luck with the tank !
I have my 55 gallon in the BASEMENT