When I worked in a pet store, I got asked that all the time. I would always answer "Water weighs 8 pounds to the gallon. Go ahead and round that up to 10 for safety. Then get however many of your friends it takes to equal that weight to stand where you want to put the tank and jump up and down a little."
:D Most folks were a bit shocked by the mental image they formed of a clown-car's worth of folks huddling. Then, they'd do the math, figure out that a good size tank is only worth about 3 sturdy guys and they'd buy the tank!