I vote for the Yote reply. It is the best suggestion for the strongest result. A cheap magnetic stud finder is less than $5 at a hardware store and running it across the ceiling below will tell you which direction the floor joists are running. A more expensive electronic one $15 will work through the floor upstairs. A 125 gallon tank with just water will run 1400 pounds, and of course the liverock and sand which will displace water is even hevier then the water, so figure 1700 pounds with sump and stand, rock etc. So consider three 265 pound men standing side by side, and that each one is carrying a 265 pound heavy man. I vote for Yotes reply.