We are adding 5 more vertical 2x4's (directly under each joist as they are staggered now) and sandwiching all the existing floor joists under the tank before I fill it. It's on an outside, load bearing, corner wall - so my builder friend told me not to worry about it, but that if I wanted to he'll help me beef it up w/ 40-50 worth of wood to make me sleep better at night.
Thankfully, as it is over the garage we had easy access to the studs/joists to do this.
*edit to add*
Tank is running across the floor joists, and the floor itself is 3/4" oak naildown.
awesome to hear. A lot of people just don't think about the floor underneath such a large tank. good luck!