water weighs 8.34lbs per gallon, so the water will weigh around 1,800lbs, plus decor and substrate, I'm guessing the tank only weighs around 100-200lbs (my glass 220 weighs 350lbs empty).
Depends on a lot. Are you positioning it near the outside wall or in the center? Parallel or perpendicular to the joists? How old is the house? What kind of stand?