Hello i'm a builder from Australia, and can tell you that over here, floor BEARERS or JOIST on second level homes are generally made from 200mm x 45mm treated pine timbers, in older houses they could be made from hard wood, which makes them even tougher.The spacing of these bearers are 450mm apart. depending on the design of the home they could run front to back or side too side of the house.Assuming your tank is on a side wall and ur bearers run side to side of the house i would be running some lengths of timber long ways across the span of your tank stand. Say ur tank is 6ft then the stand would be 6ft so the length of timber would be 6ft to span between the outer legs. This will absorb the weight more evenly in a downward motion on to ur bearers. If your bearers run front to back the span timbers from front to back of your tank stand. You can use a stud finder to locate bearer direction, or look for plaster joins or nail marks in the lower roof. I think the floor on its own would be ok to support that weight on its own, because its the same as a lower level area that has not got a concrete slab, but to be sure that would be the best way.

i know how you guys said to spread out the weight. i think the stand im getting wont have any legs? is that good?

if it has no legs and has just a spanning front and back beam then it will distribute the weight evenly on its own, like i said before if your bearers in ur floor run front of house to back of house and u have tank on side wall then planks from the front of ur stand to the back will be fine. just space them at around 350 to 450 mm apart across the length of the tank.