Will you be running perpendicular to the floor joists?
48' is crossing 3 or 4 if set just right. if you make the stand 6 feet wide, this will add one more floor joist
The end posts at an angle will not be as strong, so keep the ends perpendicular to the floor;
If you were planning on running the tank parallel to the floor joists, STOP, turn it 90 degrees cause the best you will do is 2 joists if run just right. In order to get three, then you need to make the base 35 inches & place the stand just perfect.
.. hmmm the bed falling through the floor.. is it a water bed? thats quite a bit of weight too...
With a standard stand, You are looking at 100 pounds of water weight per sq foot, plus the glass, stand & rocks.