actually you don't have to worry so much about the weight. especially if you build the stand with a solid base. that will spread the weight out a lot I did a test in my old garage before we tore it down. mind you this thing was falling apart, was not properly spaced or anything. also mind you we were a little drunk... ok.. we were hammered.
we started stacking sheets of plywood up op the rafters in the hopes of being able to pull the roof down with simple weight alone. we ran out of plywood after we had stacked 4x8 sheets of 3/4 inch plywood at a height of about 6 feet. we also ran out of room to stack. rafters were 10ft long 2x6 with minimal support. we figured out the weight to be around 3200 pounds. no issues. now, imagine a house built to standard with proper spacing and support.... theres not many houses that will have issues due to a tank. if the floor is having issues holding a tank on a proper stand, the house was already having issues before the tank was put in.