But...we recently put tile on out kitchen floor (~1000lbs). Now...my floor joists are 2x10s but like 24"OC! the contractor ended up "blocking" (I think that's the term) the joists under that area. Meaning they added blocks between the span about every 3' or so and sured up where they attached to the perimeter.
Those tiles don't weigh 40 lbs each. Consider that it is a spread load. The blocking was probably more functional for stiffening the floor support so that there would be less flex and cracking of tiles or grout. Tile likes a super stiff surface.
As long as that fish tank is on a cabinet that spreads the load across the full 9 square feet, like a water bed, you should have no problem at all. In other words don't have it standing on a table with legs. Then the pressure is far greater than 40 lbs.